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rev. a information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of analog devices. a lc 2 mos dual, complete, 12-bit/14-bit serial dacs ad7242/ad7244 functional block diagram general description the ad7242/ad7244 is a fast, complete, dual 12-bit/14-bit voltage output d/a converter. it consists of a 12-bit/14-bit dac, 3 v buried zener reference, dac output amplifiers and high speed serial interface logic. interfacing to both dacs is serial, minimizing pin count and allowing a small package size. standard control signals allow interfacing to most dsp processors and microcontrollers. asynchronous control of dac updating for both dacs is made possible with a separate ldac input for each dac. the ad7242/ad7244 operates from 5 v power supplies, providing an analog output range of 3 v. a ref out/ref in function allows the dacs to be driven from the on-chip 3 v reference or from an external reference source. the ad7242/ad7244 is fabricated in linear compatible cmos (lc 2 mos), an advanced mixed technology process that combines precision bipolar circuits with low power cmos logic. both parts are available in a 24-pin, 0.3-inch wide, plastic or hermetic dual-in-line package (dip) and in a 28-pin, plastic small outline (soic) package. the ad7242 and ad7244 are available in the same pinout to allow easy upgrade from 12-bit to 14-bit performance. features two 12-bit/14-bit dacs with output amplifiers ad7242: 12-bit resolution ad7244: 14-bit resolution on-chip voltage reference fast settling time ad7242: 3 m s to 6 1/2 lsb ad7244: 4 m s to 6 1/2 lsb high speed serial interface operates from 6 5 v supplies specified over C40 8 c to +85 8 c in plastic packages low power C 130 mw typ product highlights 1. complete, dual 12-bit/14-bit dacs the ad7242/ad7244 provides the complete function for generating voltages to 12-bit/14-bit resolution. the part features an on-chip reference, output buffer amplifiers and two 12-bit/14-bit d/a converters. 2. high speed serial interface the ad7242/ad7244 provides a high speed, easy-to-use, serial interface allowing direct interfacing to dsp processors and microcontrollers. a separate serial port is provided for each dac. 3. small package size the ad7242/ad7244 is available in a 24-pin dip and a 28- pin soic package offering considerable space saving over comparable solutions. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 617/329-4700 world wide web site: http://www.analog.com fax: 617/326-8703 ? analog devices, inc., 1996
rev. a C2C ad7242/ad7244Cspecifications (v dd = +5 v 6 5% v ss = C5 v 6 5%, agnd = dgnd = 0 v, ref ina = ref inb = +3 v. v outa , v outb load to agnd: r l = 2 k v , c l = 100 pf. all specifications t min to t max unless otherwise noted.) ad7242 parameter j, a versions 1 k, b versions 1 units test conditions/comments dc accuracy resolution 12 12 bits integral nonlinearity 1 1/2 lsb max differential nonlinearity 1 1 lsb max guaranteed monotonic bipolar zero error 5 5 lsb max positive full-scale error 2 5 5 lsb max negative full-scale error 2 5 5 lsb max reference output 3 ref out @ +25 c 2.99/3.01 2.99/3.01 v min/v max t min to t max 2.98/3.02 2.98/3.02 v min/v max ref out tempco 35 35 ppm/ c typ reference load change ( d ref out vs. d i) C1 C1 mv max reference load current change (0 m aC500 m a) reference inputs ref ina, ref inb input range 2.85/3.15 2.85/3.15 v min/v max 3 v 5% input current 1 1 m a max logic inputs ( ldaca, ldacb, tfsa, tfsb, tclka, tclkb, dta, dtb) input high voltage, v inh 2.4 2.4 v min v dd = 5 v 5% input low voltage, v inl 0.8 0.8 v max v dd = 5 v 5% input current, i in 10 10 m a max v in = 0 v to v dd input capacitance, c in 4 10 10 pf max analog outputs (v outa , v outb ) output voltage range 3 3 v nom dc output impedance 0.1 0.1 w typ short circuit current 20 20 ma typ ac characteristics 4 voltage output settling time settling time to within 1/2 lsb of final value positive full-scale change 3 3 m s max typically 2 m s negative full-scale change 3 3 m s max typically 2 m s digital-to-analog glitch impulse 10 10 nv secs typ dac code change all 1s to all 0s digital feedthrough 2 2 nv secs typ channel-to-channel isolation 110 110 db typ v out = 10 khz sine wave power requirements v dd +5 +5 v nom 5% for specified performance v ss C5 C5 v nom 5% for specified performance i dd 27 27 ma max cumulative current from the two v dd pins i ss 15 15 ma max cumulative current from the two v ss pins total power dissipation 195 195 mw max typically 130 mw notes 1 temperature ranges are as follows: j, k versions: C40 c to +85 c; a, b versions: C40 c to +85 c. 2 measured with respect to ref in and includes bipolar offset error. 3 for capacitive loads greater than 50 pf, a series resistor is required (see internal reference section). 4 sample tested @ +25 c to ensure compliance. specifications subject to change without notice. ad7242 ordering guide temperature integral package model range nonlinearity option* ad7242jn C40 c to +85 c 1 lsb max n-24 ad7242kn C40 c to +85 c 1/2 lsb max n-24 ad7242jr C40 c to +85 c 1 lsb max r-28 ad7242kr C40 c to +85 c 1/2 lsb max r-28 ad7242aq C40 c to +85 c 1 lsb max q-24 ad7242bq C40 c to +85 c 1/2 lsb max q-24 *n = plastic dip; q = cerdip; r = small outline ic (soic).
ad7244 parameter j, a versions 1 s version 1 units test conditions/comments dc accuracy resolution 14 14 bits integral nonlinearity 2 2 lsb max differential nonlinearity 1 1 lsb max guaranteed monotonic bipolar zero error 10 10 lsb max positive full-scale error 2 10 10 lsb max negative full-scale error 2 10 10 lsb max reference output 3 ref out @ +25 c 2.99/3.01 2.99/3.01 v min/v max t min to t max 2.98/3.02 2.98/3.02 v min/v max ref out tempco 35 35 ppm/ c typ reference load change ( d ref out vs. d i) C1 C1 mv max reference load current change (0 m aC500 m a) reference inputs ref ina, ref inb input range 2.85/3.15 2.85/3.15 v min/v max 3 v 5% input current 1 1 m a max logic inputs ( ldaca, ldacb, tfsa, tfsb, tclka, tclkb, dta, dtb) input high voltage, v inh 2.4 2.4 v min v dd = 5 v 5% input low voltage, v inl 0.8 0.8 v max v dd = 5 v 5% input current, i in 10 10 m a max v in = 0 v to v dd input capacitance, c in 4 10 10 pf max analog outputs (v outa , v outb ) output voltage range 3 3 v nom dc output impedance 0.1 0.1 w typ short circuit current 20 20 ma typ ac characteristics 4 voltage output settling time settling time to within 1/2 lsb of final value positive full-scale change 4 4 m s max typically 2.5 m s negative full-scale change 4 4 m s max typically 2.5 m s digital-to-analog glitch impulse 10 10 nv secs typ dac code change all 1s to all 0s digital feedthrough 2 2 nv secs typ channel-to-channel isolation 110 110 db typ v out = 10 khz sine wave power requirements v dd +5 +5 v nom 5% for specified performance v ss C5 C5 v nom 5% for specified performance i dd 27 28 ma max cumulative current from the two v dd pins i ss 15 15 ma max cumulative current from the two v ss pins total power dissipation 195 205 mw max typically 130 mw notes 1 temperature ranges are as follows: j version: 0 c to +70 c; a version: C40 c to +85 c; s version: C55 c to +125 c. 2 measured with respect to ref in and includes bipolar offset error. 3 for capacitive loads greater than 50 pf, a series resistor is required (see internal reference section). 4 sample tested @ +25 c to ensure compliance. specifications subject to change without notice. ad7242/ad7244 rev. a C3C ad7244 ordering guide temperature integral package model 1 range nonlinearity option 2 ad7244jn C40 c to +85 c 2 lsb max n-24 ad7244jr C40 c to +85 c 2 lsb max r-28 AD7244AQ C40 c to +85 c 2 lsb max q-24 ad7244sq 3 C55 c to +125 c 2 lsb max q-24 notes 1 to order mil-std-883, class b, processed parts, add /883b to part number. contact local sales office for military data sheet and availability. 2 n = plastic dip; q = cerdip; r = small outline ic (soic). 3 this grade will be available to /883b processing only.
ad7242/ad7244 rev. a C4C warning! esd sensitive device caution esd (electrostatic discharge) sensitive device. electrostatic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge without detection. although the ad7242/ad7244 feature proprietary esd protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality. timing characteristics 1, 2 limit at t min , t max limit at t min , t max parameter (j, k, a, b versions) (s version) units conditions/comments t 1 50 50 ns min tfs to tclk falling edge t 2 75 100 ns min tclk falling edge to tfs t 3 3 150 200 ns min tclk cycle time t 4 30 40 ns min data valid to tclk setup time t 5 75 100 ns min data valid to tclk hold time t 6 40 40 ns min ldac pulse width notes 1 timing specifications are sample tested at +25 c to ensure compliance. all input signals are specified with tr = tf = 5 ns (10% to 90% of 5 v) and timed from a volt- age level of 1.6 v. 2 see figure 6. 3 tclk mark/space ratio is 40/60 to 60/40. absolute maximum ratings* (t a = +25 c unless otherwise noted) v dd to agnd . . . . . . . . . . . . . . . . . . . . . . . . . C0.3 v to +7 v v ss to agnd . . . . . . . . . . . . . . . . . . . . . . . . . +0.3 v to C7 v agnd to dgnd . . . . . . . . . . . . . . . . C0.3 v to v dd + 0.3 v v out to agnd . . . . . . . . . . . . . . . . . . . . . . . . . . . . v ss to v dd ref out to agnd . . . . . . . . . . . . . . C0.3 v to v dd + 0.3 v ref ina, ref inb to agnd . . . . . . . C0.3 v to v dd + 0.3 v digital inputs to dgnd . . . . . . . . . . . . C0.3 v to v dd + 0.3 v operating temperature range j, k versions ad7244 . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 c to +70 c ad7242 . . . . . . . . . . . . . . . . . . . . . . . . . C40 c to +85 c a, b versions . . . . . . . . . . . . . . . . . . . . . . . C40 c to +85 c s version . . . . . . . . . . . . . . . . . . . . . . . . . C55 c to +125 c (v dd = +5 v 6 5%, v ss = C5 v 6 5%, agnd = dgnd = 0 v) pin configurations storage temperature range . . . . . . . . . . . . C65 c to +150 c lead temperature (soldering, 10 sec) . . . . . . . . . . . . +300 c power dissipation (any package) to +75 c . . . . . . . 550 mw derates above +75 c by . . . . . . . . . . . . . . . . . . . . . 6 mw/ c *stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only, functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. dip soic
ad7242/ad7244 rev. a C5C ad7242/ad7244 pin function description dip pin no. mnemonic description 1 ldaca load dac, logic input. a new word is transferred into dac latch a from input latch a on the fall- ing edge of this signal. if ldaca is hard-wired low, data is transferred from input latch a to dac latch a on the sixteenth falling edge of tclka after tfsa goes low. 2 tfsa transmit frame synchronization, logic input. this is a frame or synchronization signal for daca data with serial data expected after the falling edge of this signal. 3 dta transmit data, logic input. this is the data input which is used in conjunction with tfsa and tclka to transfer serial data to input latch a. 4 tclka transmit clock, logic input. serial data bits for daca are latched on the falling edge of tclka when tfsa is low. 5 dgnd digital ground. both dgnd pins for the device must be tied together at the device. 6 tp1 test pin 1. used when testing the device. do not connect anything to this pin. 7v dd positive power supply, 5 v 5%. both v dd pins for the device must be tied together at the device. 8 agnd analog ground. both agnd pins for the device must be tied together at the device. 9v outb analog output voltage from dacb. this output comes from a buffer amplifier. the range is bipolar, 3 v with ref inb = +3 v. 10 v ss negative power supply, C5 v 5%. both v ss pins for the device must be tied together at the device. 11 tp2 test pin 2. used when testing the device. do not connect anything to this pin. 12 ref inb dacb voltage reference input. the voltage reference for dacb is applied to this pin. it is internally buffered before being applied to dacb. the nominal reference voltage for correct operation of the ad7242/ad7244 is 3 v. 13 ldacb load dac, logic input. a new word is transferred into dac latch b from input latch b on the fall- ing edge of this signal. if ldacb is hard-wired low, data is transferred from input latch b to dac latch b on the sixteenth falling edge of tclkb after tfsb goes low. 14 tfsb transmit frame synchronization, logic input. this is a frame or synchronization signal for dacb data with serial data expected after the falling edge of this signal. 15 dtb transmit data, logic input. this is the data input used in conjunction with tfsb and tclkb to transfer serial data to input latch b. 16 tclkb transmit clock, logic input. serial data bits for dacb are latched on the falling edge of tclkb when tfsb is low. 17 dgnd digital ground. both dgnd pins for the device must be tied together at the device. 18 tp3 test pin 3. used when testing the device. do not connect anything to this pin. 19 v dd positive power supply, 5 v 5%. both v dd pins for the device must be tied together at the device. 20 agnd analog ground. both agnd pins for the device must be tied together at the device. 21 v outa analog output voltage from daca. this output comes from a buffer amplifier. the range is bipolar, 3 v with ref ina = +3 v. 22 v ss negative power supply, C5 v 5%. both v ss pins for the device must be tied together at the device. 23 ref out voltage reference output. to operate the dacs with this internal reference, ref out should be connected to both ref ina and ref inb. the external load capability of the reference is 500 m a. 24 ref ina daca voltage reference input. the voltage reference for daca is applied to this pin. it is internally buffered before being applied to daca. the nominal reference voltage for correct operation of the ad7242/ad7244 is 3 v.
ad7242/ad7244 rev. a C6C circuit description the ad7242/ad7244 contains two 12-bit/14-bit d/a convert- ers, each with an output buffer amplifier. the part also contains a reference input buffer amplifier for each dac, and an on-chip 3 v reference. d/a section the ad7242/ad7244 contains two 12-bit/14-bit voltage mode d/a converters, each consisting of highly stable thin-film resistors and high speed single-pole, double-throw switches. the simplified circuit diagram for the dac section is shown in figure 1. the three msbs of the data word are decoded to drive the seven switches a-g. on the ad7242, the 9 lsbs switch a 9-bit r-2r ladder structure while on the ad7244, the 11 lsbs switch an 11-bit r-2r ladder structure. the output voltage from this converter has the same polarity as the reference voltage, ref in. the ref in voltage is internally buffered by a unity gain amplifier before being applied to the d/a converters and the bipolar bias circuitry. the d/a converter is configured and scaled for a 3 v reference, and the device is tested with 3 v applied to ref in. operating the ad7242/ad7244 at refer- ence voltages outside the 5% tolerance range may result in degraded performance from the part. figure 1. dac ladder structure internal reference the on-chip reference is a temperature-compensated buried zener reference that is factory trimmed for 3 v 10 mv. the reference can be used to provide both the reference voltage for the two d/a converters and the bipolar biasing circuitry. this is achieved by connecting ref out to ref ina and ref inb. the reference voltage can also be used for other components and is capable of providing up to 500 m a to an external load. the maximum recommended capacitance on the reference output pin for normal operation is 50 pf. if the reference output is required to drive a capacitive load greater than 50 pf, a 200 w resistor should be placed in series with the capacitive load. decoupling the ref out pin with a series 200 w resistor and a parallel combination of a 10 m f tantalum capacitor and a 0.1 m f ceramic capacitor as in figure 2 reduces the noise spectral density of the reference (see figure 4). using this decoupling scheme to generate the reference voltage for ref ina and ref inb gives a channel-to-channel isolation number of 110 db (connecting ref out directly to ref ina and ref inb gives 80 db). the channel-to-channel isolation is 110 db using an external reference. external reference in some applications, the user may require a system reference or some other external reference to drive the ad7242/ad7244 reference inputs. figure 3 shows how the ad586 reference can be conditioned to provide the 3 v reference required by the ad7242/ad7244 reference inputs. figure 2. circuit connection for ref out with an external capacitive load of greater than 50 pf figure 3. ad586 driving ad7242/ad7244 reference inputs
ad7242/ad7244 rev. a C7C output amplifier the outputs from each of the voltage-mode dacs are buffered by a noninverting amplifier. the buffer amplifier is capable of developing 3 v across a 2 k w and 100 pf load to ground, and can produce 6 v peak-to-peak sine wave signals to a frequency of 20 khz. the output is updated on the falling edge of the respective ldac input. the output voltage settling time, to within 1/2 lsb of its final value, is typically less than 2 m s for the ad7242 and 2.5 m s for the ad7244. the small signal (200 mv p-p) bandwidth of the output buffer amplifier is typically 1 mhz. the output noise from the amplifier is low, with a figure of 30 nv/ ? hz at a frequency of 1 khz. the broadband noise from the amplifier exhibits a typical peak-to-peak figure of 150 m v for a 1 mhz output bandwidth. figure 4 shows a typical plot of noise spectral density versus frequency for the output buffer amplifier and for the on-chip reference (including and excluding the decoupling components). figure 4. noise spectral density vs. frequency transfer function the basic circuit configuration for the ad7242/ad7244 is shown in figure 5. table i and table ii show the ideal input code to output voltage relationship for the ad7242 and ad7244 respectively. input coding for the ad7242/ad7244 is 2s complement. figure 5. basic connection diagram for the ad7242, the output voltage can be expressed in terms of the input code, n , using the following relationship: v out = 2 n ref in 4096 where C2048 n +2047 for the ad7244, the output voltage can be expressed in terms of the input code, n , using the following relationship: v out = 2 n ref in 16384 where C8192 n +8191 table i. ad7242 ideal input/output code table code dac latch contents msb lsb analog output, v out * 01 11 1111 1111 +2.998535 v 01 11 1111 1110 +2.99707 v 00 00 0000 0001 +0.001465 v 00 00 0000 0000 0 v 11 11 1111 1111 C0.001465 v 10 00 0000 0001 C2.998535 v 10 00 0000 0000 C3 v *assuming ref in = +3 v. table ii. ad7244 ideal input/output code table code dac latch contents msb lsb analog output, v out * 01 1111 1111 1111 +2.999634 v 01 1111 1111 1110 +2.99268 v 00 0000 0000 0001 +0.000366 v 00 0000 0000 0000 0 v 11 1111 1111 1111 C0.000366 v 10 0000 0000 0001 C2.999634 v 10 0000 0000 0000 C3 v *assuming ref in = +3 v.
ad7242/ad7244 rev. a C8C timing and control communication with the ad7242/ad7244 is via six serial logic inputs. these consist of separate serial clocks, word framing and data lines for each dac. dac updating is controlled by two digital inputs: ldaca for updating v outa and ldacb for updating v outb . these inputs can be asserted independently of the microprocessor by an external timer when precise updating intervals are required. alternatively, the ldaca and ldacb inputs can be driven from a decoded address bus allowing the microprocessor control over dac updating as well as data communication to the ad7242/ad7244 input latches. the ad7242/ad7244 contains two latches per dac, an input latch and a dac latch. data must be loaded to the input latch under the control of tclka, tfsa and dta for input latch a and tclkb, tfsb and dtb for input latch b. data is then transferred from input latch a to dac latch a under the control of the ldaca signal, while ldacb controls the loading of dac latch b from input latch b. only the data held in the dac latches determines the analog outputs of the ad7242/ad7244. data is loaded to the input latches under control of the respec- tive tclk, tfs and dt signals. the ad7242/ad7244 expects a 16-bit stream of serial data on its dt inputs. data must be valid on the falling edge of tclk. the tfs input provides the frame synchronization signal that tells the ad7242/ ad7244 that valid serial data will be available on the dt input for the next 16 falling edges of tclk. figure 6 shows the timing diagram for operation of either of the two serial input ports on the part. although 16 bits of data are clocked into the input latch, only 12 bits are transferred into the dac latch for the ad7242 and 14 bits are transferred for the ad7244. therefore, 4 bits in the ad7242 data stream and 2 bits in the ad7244 data stream are dont cares since their value does not affect the dac latch data. the bit positions are the dont cares followed by the dac data starting with the msb (see figure 6). the respective ldac signals control the transfer of data to the respective dac latches. normally, data is loaded to the dac latch on the falling edge of ldac . however, if ldac is held low, serial data is loaded to the dac latch on the sixteenth falling edge of tclk. if ldac goes low during the loading of serial data to the input latch, no dac latch update takes place on the falling edge of ldac . if ldac stays low until the serial transfer is completed, then the update takes place on the sixteenth falling edge of tclk. if ldac returns high before the serial data transfer is completed, no dac latch update takes place. if seventeen or more tclk edges occur while tfs is low, the seventeenth (and beyond) clock edges are ignored, i.e., no further data is clocked into the input latch after the sixteenth tclk edge following a falling edge on tfs . figure 6. ad7242/ad7244 timing diagram
ad7242/ad7244 rev. a C9C microprocessor interfacing microprocessor interfacing to the ad7242/ad7244 is via a serial bus that uses standard protocol compatible with dsp processors and microcontrollers. the communication interface consists of a separate transmit section for each of the dacs. each section has a clock signal, a data signal and a frame or strobe pulse. figures 7 through 11 show the ad7242/ad7244 configured for interfacing to a number of popular dsp processors and microcontrollers. ad7242/ad7244 to adsp-2101/adsp-2102 interface figure 7 shows a serial interface between the ad7242/ad7244 and the adsp-2101/adsp-2102 dsp processor. the adsp- 2101/adsp-2102 has two serial ports and, in the interface shown, both serial ports are used, one for each dac. both serial ports do not have to be used; in the case where only one serial port is used, an extra line (daca/ dacb as shown in the other serial interfaces) would have to decode the one tfs line to provide tfsa and tfsb lines for the ad7242/ad7244. figure 7. ad7242/ad7244 to adsp-2101/adsp-2102 interface the three serial lines of the first serial port, sport1, of the adsp-2101/adsp-2102 connect directly to the three serial input lines of daca of the ad7242/ad7244. the three serial lines of sport2 connect directly to the three serial lines on the dacb serial input port. data from the adsp-2101/adsp-2102 is valid on the falling edge of sclk. a common ldac signal is used to drive the ldaca and ldacb inputs. this is shown to be generated from a timer or clock recovery circuit but another control or address line of the adsp-2101/adsp-2102 could be used to drive these inputs. alternatively, the ldaca and ldacb inputs of the ad7242/ad7244 could be hardwired low; in this case the update of the dac latches and analog outputs takes place on the 16th falling edge of sclk (after the respective tfs signal goes low). ad7242/ad7244 to dsp56000 interface a serial interface between the ad7242/ad7244 and the dsp56000 is shown in figure 8. the dsp56000 is configured for normal mode, asynchronous operation with gated clock. it is also set up for a 16-bit word with sck and sc2 as outputs and the fsl control bit set to a 0. sck is internally generated on the dsp56000 and applied to both the tclka and tclkb inputs of the ad7242/ad7244. data from the dsp56000 is valid on the falling edge of sck. the serial data line, std drives the dta and dtb serial input data lines of the ad7242/ad7244. the sc2 output provides the framing pulse for valid data. this is an active high output and is gated with a daca/ dacb control line before being applied to the tfsa and tfsb inputs of the ad7242/ad7244. the daca/ dacb line determines which dac serial data is to be transferred to, i.e., which tfs line is active when sc2 is active. as in the previous interface, a common ldac input is shown driving the ldaca and ldacb inputs of the ad7242/ad7244. once again, these ldac inputs could be hardwired low, in which case v outa or v outb will be updated on the sixteenth falling edge of sck after the tfsa or tfsb input goes low. figure 8. ad7242/ad7244 to dsp56000 interface
ad7242/ad7244 rev. a C10C ad7242/ad7244 to tms320c25 interface figure 9 shows a serial interface between the ad7242/ad7244 and the tms320c25 dsp processor. in this interface, the clkx and fsx signals of the tms320c25 are generated from the clock/timer circuitry. the fsx pin of the tms320c25 must be configured as an input. clkx is used to provide both the tclka and tclkb inputs of the ad7242/ad7244. dx of the tms320c25 is also routed to the serial data line of each input port of the ad7242/ad7244. data from the tms32020 is valid on the falling edge of clkx after fsx goes low. this fsx signal is gated with the daca/ dacb control line to determine whether tfsa or tfsb goes low when fsx goes low. the clock/timer circuitry also generates the ldac signal for the ad7242/ad7244 to synchronize the update of the outputs with the serial transmission. as in the previous interface diagrams, a common ldac input is shown driving the ldaca and ldacb inputs of the ad7242/ad7244. once again, these ldac inputs could be hardwired low, in which case v outa or v outb will be updated on the sixteenth falling edge of clkx after the tfsa or tfsb input goes low. figure 9. ad7242/ad7244 to tms320c25 interface ad7242/ad7244 to 87c51 interface a serial interface between the ad7242/ad7244 and the 87c51 microcontroller is shown in figure 10. txd of the 87c51 drives tclka and tclkb of the ad7242/ad7244 while rxd drives the two serial data lines of the part. the tfsa and tfsb signals are derived from p3.2 and p3.3, respectively. the 87c51 provides the lsb of its sbuf register as the first bit in the serial data stream. therefore, the user will have to ensure that the data in the sbuf register is correctly arranged so the dont care bits are the first to be transmitted to the ad7242/ ad7244; the last bit to be sent is the lsb of the word to be loaded to the ad7242/ad7244. when data is to be transmitted to the part, p3.2 (for daca) or p3.3 (for dacb) is taken low. data on rxd is valid on the falling edge of txd. the 87c51 transmits its serial data in 8-bit bytes with only eight falling clock edges occurring in the transmit cycle. to load data to the ad7242/ad7244, p3.2 (for daca) or p3.3 (for dacb) is left low after the first eight bits are transferred and a second byte of data is then serially transferred to the ad7242/ad7244. when the second serial transfer is complete, the p3.2 line (for daca) or the p3.3 line (for dacb) is taken high. figure 10 shows both ldac inputs of the ad7242/ad7244 hardwired low. as a result, the dac latch and the analog output of one of the dacs will be updated on the sixteenth falling edge of txd after the respective tfs signal for that dac has gone low. alternatively, the scheme used in previous interfaces, whereby the ldac inputs are driven from a timer, can be used. figure 10. ad7242/ad7244 to 87c51 interface ad7242/ad7244 to 68hc11 interface figure 11 shows a serial interface between the ad7242/ad7244 and the 68hc11 microcontroller. sck of the 68hc11 drives tclka and tclkb of the ad7242/ad7244 while the mosi output drives the two serial data lines of the ad7242/ad7244. the tfsa and tfsb signals are derived from pc6 and pc7, respectively. for correct operation of this interface, the 68hc11 should be configured such that its cpol bit is a 0 and its cpha bit is a 1. when data is to be transmitted to the part, pc6 (for daca) or pc7 (for dacb) is taken low. when the 68hc11 is configured like this, data on mosi is valid on the falling edge of sck. the 68hcll transmits its serial data in 8-bit bytes with only eight falling clock edges occurring in the transmit cycle. to load data to the ad7242/ad7244, pc6 (for daca) or pc7 (for dacb) is left low after the first eight bits are transferred and a second byte of data is then serially transferred to the ad7242/ad7244. when the second serial transfer is complete, the pc6 line (for daca) or the pc7 line (for dacb) is taken high. figure 11. ad7242/ad7244 to 68hc11 interface figure 11 shows both ldac inputs of the ad7242/ad7244 hardwired low. as a result, the dac latch and the analog output of one of the dacs will be updated on the sixteenth falling edge of sck after the respective tfs signal for that dac has gone low. alternatively, the scheme used in previous interfaces, whereby the ldac inputs are driven from a timer, can be used.
ad7242/ad7244 rev. a C11C applying the ad7242/ad7244 good printed circuit board layout is as important as the overall circuit design itself in achieving high speed converter perfor- mance. the ad7242 works on an lsb size of 1.465 mv, while the ad7244 works on an lsb size of 366 m v. therefore, the designer must be conscious of minimizing noise in both the converter itself and in the surrounding circuitry. switching mode power supplies are not recommended as the switching spikes can feed through to the on-chip amplifier. other causes of concern are ground loops and digital feedthrough from microprocessors. these are factors that influence any high performance converter, and a proper pcb layout that minimizes these effects is essential for best performance. layout hints ensure that the layout for the printed circuit board has se parated digital and analog lines as much as possible. take care not to run any digital track alongside an analog signal track. establish a single point analog ground (star ground) separate from the logic system ground. place this star ground as close as possible to the ad7242/ad7244. connect all analog grounds to this star ground and also connect the ad7242/ad7244 dgnd pins to this ground. do not connect any other digital grounds to this analog ground point. low impedance analog and digital power supply common returns are essential to low noise operation of high performance converters. therefore, the foil width for these tracks should be kept as wide as possible. the use of ground planes minimizes impedance paths and also guards the analog circuitry from digital noise. noise keep the signal leads on the v outa and v outb signals and the signal return leads to agnd as short as possible to minimize noise coupling. in applications where this is not possible, use a shielded cable between the dac outputs and their destination. reduce the ground circuit impedance as much as possible since any potential difference in grounds between the dac and its destination device appears as an error voltage in series with the dac output.
ad7242/ad7244 rev. a C12C outline dimensions dimensions shown in inches and (mm). c1421C10C10/90 printed in u.s.a. plastic dip (n-24) cerdip (q-24) soic (r-28)

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