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Preliminary Data Sheet
FEATURES
Low Noise Voltage noise = 2.2 nV/Hz Current noise = 2.5 pA/Hz (Positive Input) Wide Bandwidth (-3 dB) = 250 MHz Nominal Gain Range 0 dB to 24 dB (Preamp Gain = 6 dB) Gain Scaling 20 dB/V DC Coupled Single Ended Input and Output Supplies: +5V, +/-2.5V or +/-5V Low power: 78 mW
250MHz General Purpose Low Cost, DC Coupled VGA AD8337
FUNCTIONAL BLOCK DIAGRAM
VPOS 8 GAIN CONTROL INTERFACE
GAIN 7
1 OUTP
INPP 3 INPN 4 VCOM 2 5 PRAO 6 VNEG
APPLICATIONS
Gain Trim PET Scanners High performance AGC systems I/Q signal processing Video Industrial and Medical Ultrasound Radar Receivers
Figure 1. AD8337 Functional Block Diagram and Pinout
GENERAL DESCRIPTION
The AD8337 is a low noise single ended linear-in-dB, general purpose variable gain amplifier usable as a low noise variable gain element at frequencies up to 100 MHz; the -3 dB bandwidth is 250 MHz. The topology used is an X-AMP(R) structure with 24 dB of gain range. The VGA is intended for trim applications. Excellent bandwidth uniformity is maintained across the entire range. The gain control interface provides precise linear-in-dB scaling of 20 dB/V and is centered on VCOM. The VGA's low outputreferred noise is advantageous in driving high speed ADCs. Excellent DC characteristics and high speed make the AD8337 particularly suited for industrial ultrasound, PET scanners, and for video applications. Dual supply operation enables gain control of negative-going pulses such as generated by photodiodes or photo-multiplier tubes. The AD8337 contains an operational amplifier (preamplifier - PrA) at its input which can be configured for any gain greater than two with external resistors, this allows both inverting and non-inverting topologies and thereby a dual polarity VGA. The VGA is specified with a non-inverting PrA gain-of-2. The attenuator has a range of 24 dB and the output amplifier has a gain-of-8 (18.06 dB). Because the preamplifier gain can be set through external resistors, the gain range will shift up or down depending on the PrA gain, nominally the gain range is from 0 to +24 dB. For larger gain ranges, multiple VGAs can be connected in series. This also allows for interstage filtering to suppress noise and distortion. The operating temperature range is -40C to +85. The AD8337 is available in a 3x3 mm 8 pin chip-scale package (CSP).
Rev. PrA 4/26/2005 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 that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.326.8703 (c) 2005 Analog Devices, Inc. All rights reserved.
AD8337
Preliminary Data Sheet
TABLE OF CONTENTS
AD8337--Specifications.................................................................. 3 Absolute Maximum Ratings............................................................ 5 Pin Configuration and Functional Descriptions.......................... 6 ESD CAUTION ............................................................................ 6 Theory Of Operation ....................................................................... 7 Preamplifier................................................................................... 7 VGA................................................................................................ 7 Gain Control ................................................................................. 7 Attenuator.......................................................................................7 Output Stage...................................................................................7 Single Supply Operation/ AC coupling ......................................8 Noise ...............................................................................................8 Applications........................................................................................9 Driving Capacitive Loads.............................................................9 Board Layout..................................................................................9
REVISION HISTORY
Rev. PrA:
Rev. PrA | Page 2 of 9
Preliminary Data Sheet AD8337--SPECIFICATIONS
AD8337
Table 1. VS = 2.5 V, TA = 25C, PrA Gain = +2 (RFB1 = RFB2 = 100 ), VCOM = GND; f = 10 MHz, CL = 10 pF, RL = 500 , unless otherwise specified.
Parameter GENERAL PARAMETERS -3 dB Small Signal Bandwidth -3 dB Large Signal Bandwidth Slew Rate Input Voltage Noise Input Current Noise Noise Figure Output-Referred Noise Output Impedance Output Signal Range Output Offset Voltage DYNAMIC PERFORMANCE Harmonic Distortion HD2 HD3 HD2 HD3 HD2 HD3 Output 1 dB Compression Point Two-Tone Intermodulation Distortion (IMD3) Conditions VOUT = 10 mV p-p VOUT = 1 V p-p VOUT = 2 V p-p square wave VOUT = 1 V p-p square wave f = 10 MHz f = 10 MHz VGAIN = 0.7 V, RS = 50 , unterminated VGAIN = 0.7 V, RS = 1k, unterminated VGAIN = 0.7 V (Gain = 24 dB) VGAIN = -0.7 V (Gain = 0 dB) DC to 10 MHz RL 500 , Vsupply = 2.5 V, + 5V RL 500 , Vsupply = 5 V VGAIN = 0.7 V (Gain = 24 dB) VS = 2.5V VGAIN = 0V, VOUT = 1 Vpp f = 1 MHz f = 10 MHz f = 45 MHz VGAIN = -0.7 V, f = 10 MHz VGAIN = +0.7 V, f = 10 MHz VGAIN = 0 V, VOUT = 1 Vpp, f1 = 10 MHz, f2 = 11 MHz VGAIN = 0, VOUT = 1 Vpp, f1 = 45 MHz, f2 = 46 MHz VGAIN = 0, VOUT = 2 Vpp, f1 = 10 MHz, f2 = 11 MHz VGAIN = 0, VOUT = 2 Vpp, f1 = 45 MHz, f2 = 46 MHz VGAIN = 0, VOUT = 1 Vpp, f = 10 MHz VGAIN = 0, VOUT = 1 Vpp, f = 45 MHz VGAIN = 0, VOUT = 2 Vpp, f = 10 MHz VGAIN = 0, VOUT = 2 Vpp, f = 45 MHz VGAIN = 0.7 V, VIN = 50 - 500 mV p-p 1 MHz < f < 100 MHz, Full Gain Range VS = 5V VGAIN = 0V, VOUT = 1 Vpp f = 1 MHz f = 10 MHz f = 45 MHz VGAIN = -0.7 V, f = 10 MHz VGAIN = +0.7 V, f = 10 MHz VGAIN = 0 V, VOUT = 1 Vpp, f1 = 10 MHz, f2 = 11 MHz Min Typ1 250 190 475 375 2.2 2.5 8 2 34 17.5 0.7+j1.3 VCOM 1.4 VCOM 3.4 <20 Max Unit MHz MHz V/s V/s nV/Hz pA/Hz dB dB nV/Hz nV/Hz V V mV
TBD
TBD
Output Third Order Intercept
Overload Recovery Group Delay Variation DYNAMIC PERFORMANCE Harmonic Distortion HD2 HD3 HD2 HD3 HD2 HD3 Output 1 dB Compression Point Two-Tone Intermodulation
1 2
-64 -66 -63 -61 -61 -72 +9.5 +15.8 -72 -58 -61 -45 33 26 34 26 50 TBD
dBc dBc dBc dBc dBc dBc dBm2 dBm dBc dBc dBc dBc dBm dBm dBm dBm ns ns
-68 -68 -73 -65 -63 -69 15.8 23.4 -75
dBc dBc dBc dBc dBc dBc dBm dBm dBc
Bold/Italic values are measured, all others are simulated and still need to be confirmed. All dBm values are calculated with 50 reference, unless otherwise noted. Rev. PrA | Page 3 of 9
AD8337
Parameter Distortion (IMD3) Conditions VGAIN = 0, VOUT = 1 Vpp, f1 = 45 MHz, f2 = 46 MHz VGAIN = 0, VOUT = 2 Vpp, f1 = 10 MHz, f2 = 11 MHz VGAIN = 0, VOUT = 2 Vpp, f1 = 45 MHz, f2 = 46 MHz VGAIN = 0, VOUT = 1 Vpp, f = 10 MHz VGAIN = 0, VOUT = 1 Vpp, f = 45 MHz VGAIN = 0, VOUT = 2 Vpp, f = 10 MHz VGAIN = 0, VOUT = 2 Vpp, f = 45 MHz VGAIN = 0.7 V, VIN = 0.1 - 1 V p-p 1 MHz < f < 100 MHz, Full Gain Range -0.7 V < VGAIN < -0.6 V -0.6 V < VGAIN < 0.6 V 0.6 V < VGAIN < 0.7 V -0.5 V < VGAIN < 0.5 V
Preliminary Technical Data
Min Typ1 -60 -64 -48 34 27 35 27 TBD TBD TBD TBD TBD TBD 0.25 TBD TBD 24 20 12.5 -VS TBD TBD |4.5| |5| 15.5 78 -35 18.5 185 TBD |10| VS TBD TBD TBD Max Unit dBc dBc dBc dBm dBm dBm dBm ns ns dB dB dB dB dB dB/V dB V M ns V mA mW dB mA mW dB
Output Third Order Intercept
Overload Recovery Group Delay Variation ACCURACY Absolute Gain Error3
Gain Law Conformance4 GAIN CONTROL INTERFACE Gain Range Gain Scaling Factor Gain Intercept Input Voltage (VGAIN) Range Input Impedance Response Time POWER SUPPLY Supply Voltage Quiescent Current Power Dissipation PSRR Quiescent Current Power Dissipation PSRR
VGAIN = 0 V No foldover 24 dB Gain Change VPOS - VNEG (dual and single supply operation) Each Supply (VPOS and VNEG) No Signal, VPOS-VNEG = 5V VGAIN = 0.7 V, f = 10 MHz Vsupply = 5V Each Supply (VPOS and VNEG) No Signal, VS = 5V VGAIN = 0.7 V, f = 10 MHz
3 4
Conformance to theoretical gain expression (see Equation 1). Conformance to best fit dB linear curve.
Rev. PrA | Page 4 of 9
Preliminary Data Sheet ABSOLUTE MAXIMUM RATINGS
Table 2. Absolute Maximum Ratings
Parameter Voltage Supply Voltage (VPOS, VNEG) Input Voltage (INPx) GAIN Voltage Power Dissipation Temperature Operating Temperature Storage Temperature Lead Temperature (Soldering 60 sec) JA xx Package5
5
AD8337
Rating 6V TBD V VPOS, VNEG TBD W -40C to +85C -65C to +150C 300C TBDC/W
Stresses above those listed under the 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 indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
Four-Layer JEDEC Board (2S2P).
Rev. PrA | Page 5 of 9
AD8337 PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS
VOUT VCOM INPP INPN 1 2 3 4
Pin 1 Identifier
Preliminary Technical Data
Pin No. 1 2 3 4 5 6 7 Mnemonic VOUT VCOM INPP INPN PRAO VNEG GAIN VPOS Function VGA Output Common - Ground for Dual Supply; Apply VPOS/2 for Single Supply Positive Input to Preamplifier Negative Input to Preamplifier Preamplifier Output Negative Supply (-VPOS for Dual Supply; GND for Single Supply) Gain Control Input centered at VCOM Positive Supply
8 7 6 5
VPOS GAIN VNEG PRAO
AD8337
Top View (not to scale)
8
Figure 1. 8 LFCSP
Table 3. Pin Function Descriptions
ESD 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 this product features 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.
Rev. PrA | Page 6 of 9
Preliminary Data Sheet THEORY OF OPERATION
The AD8337 is a low noise single ended linear-in-dB, general purpose variable gain amplifier (VGA) usable as a low noise variable gain element at frequencies up to 100 MHz; the -3 dB bandwidth is 250 MHz. It is fabricated in an ADI proprietary dielectrically isolated complementary bipolar process with fTs in the 3- 5 GHz range. The part is DC coupled throughout and has relatively low offset even at maximum gain. The power consumption is very low at only 78 mW on a 5 V supply (either single +5 V or 2.5 V); the supply current is typically about15.5 mA in this case. Figure 2 shows the circuit block diagram of the AD8337.
Preamp 6dB (nominally)
INPP INPL Rfb1 Rfb2 VPOS VNEG
AD8337
Gain(dB ) = 20 dB Vgain + ICPT (dB) , V
where the nominal intercept (ICPT) is 12.15 dB. If the gain of the preamp is increased then ICPT will increase accordingly. For example, if the gain of the preamp is increased by 6 dB, then ICPT will shift up by 6 dB to 18.15 dB.
GAIN CONTROL
The gain control interface provides a high impedance input and is referenced to pin VCOM which for maximum output swing should be always mid-supply at (VPOS+VNEG)/2. In a dual supply configuration, as in the low power configuration with 2.5 V supplies, this would be ground. The voltage on pin VCOM determines the midpoint of the gain range which normally runs from -0.7 to +0.7 V with the most linear-in-dB section of the gain control running from about -0.6 to +0.6 V. In this middle section the gain error is typically less than 0.2 dB. The gain control voltage can be increased or decreased to the positive and negative supply voltages without gain foldover. The gain scaling factor (gain slope) is 20 dB/V, this relatively low slope ensures that noise on the GAIN input will not be unduly amplified. Since a VGA is really a multiplier it is important to make sure that the GAIN input does not accidentally modulate the output signal because of some unwanted signal coupling onto the gain control line. Because of the high input impedance of the GAIN input it is easy to add a simple low pass filter to eliminate any unwanted signal on the gain control input.
Rs
+ PrA -
PRAO
Outamp 18dB (x8) + Attenuator -24 to 0 dB +
VOUT
-
R2 = 749 R1 = 107
Interpolator
GAIN Interface
BIAS
VCOM GAIN
Figure 2.
Block Diagram of the AD8337
PREAMPLIFIER
The AD8337 also includes a current feedback preamplifier that buffers the ladder network of the X-AMP(R). External resistor can be used to set the gain of the preamplifier but it is specified with a non-inverting gain-of-2 with Rfb1 = Rfb2 = 100 . Other gains can be implemented, however, it is important to not use a smaller value for Rfb2 than 100 because Rfb2 along with an internal compensation capacitor determines the 3 dB bandwidth of the preamplifier. If a smaller resistor is used, the preamplifier may go unstable. Larger values for Rfb2 will reduce the bandwidth and increase the preamplifier gain; this allows the user to shift the 24 dB gain range up from the nominal 0 - 24 dB range. Increasing the preamplifier gain will increase the offset because of the DC coupling. The only other thing to watch out for is that if Rfb1 also is increased then the input referred noise will increase, otherwise the preamplifier can be thought of as an uncommitted op-amp that is greater than gain-of-2 stable.
ATTENUATOR
The attenuator in the VGA presents an input resistance of nominally 265 s to the preamplifier, this together with the external 200 s of Rfb1 and Rfb2 results in an effective load of about 114 that the preamplifier has to drive. The attenuator is made up of eight -3.01 dB sections for a total attenuation range of -24.08 dB. Following the attenuator is a fixed gain amplifier with x8 gain. Because of this relatively low gain, the output offset is kept well below 20 mV even over temperature; the offset is largest at max gain since then the preamplifier offset is amplified. The VCOM pin defines the common-mode reference for the output as seen in Figure 2.
VGA
The VGA is a standard X-AMP architecture which has a linearin-dB gain characteristic. This architecture has been proven to give the best output dynamic range in receive applications. As can be seen in Figure 2, the variable attenuator range is 24 dB, this is then followed by a fixed gain amplifier of 18 dB for a total VGA gain range of -6 dB to +18 dB. Together with the preamplifier configured with a gain of 6 dB, this results in the specified gain range of 0 to +24 dB. The VGA plus preamp with 6 dB of gain implements a gain law as follows:
OUTPUT STAGE
The output stage of the VGA is similar to the preamplifier and is very high speed. It is a Class AB complementary emitter follower type output stage; these stages tend to look inductive (increasing impedance) with increasing frequency because of the AC-beta roll-off of the output devices and the inherent reduction in feedback beyond the -3 dB point. High speed output stages like this tend to be able to drive large currents, however, they are also more susceptible to capacitive loading. A small series resistor can reduce the effects of capacitive loading (see Application section).
Rev. PrA | Page 7 of 9
AD8337
SINGLE SUPPLY OPERATION/ AC COUPLING
If a user wants to run the AD8337 from a single +5 V supply, then VCOM needs to be biased from a good 2.5 V reference, especially if the part is still DC coupled; the voltage source applied to VCOM needs to be able to handle the currents that flow in the load of the preamplifier and the output stage of the VGA. In the event that a user wants to AC couple the AD8337, it is essential that some bias network is provided to pin INPP. In that case the bias generator for pin VCOM "only" needs to be able to supply the dynamic current to the preamplifier feedback network and the gain setting resistors of the VGA. For most single +5 V supply applications, if no +2.5 V supply is available, it will be necessary to use a good op-amp with enough current drive capability and possibly a reference like the ADR431.
Preliminary Technical Data
low gains, this result divided by the VGA fixed gain amplifier gain of x8, results in about 1.9 nV/Hz referred to the VGA input. Note that this value includes the noise of the VGA gain setting resistors as well. If this voltage is again divided by the preamp gain-of-2, then the VGA noise referred all the way to the preamp input is about 0.8 nV/Hz. From this we can determine that the preamplifier, including the 100 gain setting resistors, contribute about 2 nV/Hz. The two 100 resistors contribute each 1.29 nV/Hz at the output of the preamp, when this is divided down by 2 and subtracted, then the preamplifier noise can be determined at about 1.75 nV/Hz. The following equation shows the calculation that determines the output referred noise at max gain (24 dB or x16): At = total gain from preamp input to VGA output; RS = source resistance; en-PrA = input referred voltage noise of preamp; in-PrA = current noise of preamp at INPP pin; en-Rfb1 = voltage noise of Rfb1; enRfb2 = voltage noise of Rfb2; en-VGA = input referred voltage noise of VGA (low gain output referred noise divided by fixed gain of x8).
NOISE
The total input referred noise is about 2.2 nV/Hz and the current noise about 2.5 pA/Hz on the positive preamp input, INPP. The VGA output referred noise is about 15 nV/Hz at
en - out =
(RS At )2 + (en - Pr A At )2 + (in - Pr A RS )2 + (en - Rfb1 Rfb2 AVGA )2 + (en - Rfb 2 AVGA )2 + (en -VGA AVGA )2
Rfb1
This simplifies to the following if RS = 0, Rfb1 = Rfb2 = 100 , At = 16, AVGA = 8.
en - out = (1.75 16) 2 + 2 (1.29 8) 2 + (1.9 8) 2 = 35nV / Hz .
Taking this result and dividing by 16 gives the total input referred noise with a short circuited input as 2.2 nV/Hz. When the preamplifier is used in the inverting configuration with the same Rfb1 and Rfb2 = 100 as above then en-out does not change, however, because the gain dropped by 6 dB, the input referred noise increases by a factor-of-2 to about 4.4 nV/Hz. The reason for this is that the noise gain to the output of all the noise generators stays the same, yet the preamp in the inverting configuration has a gain of (-1) compared to the (+2) in the non-inverting configuration; this increases the input referred noise by two.
Rev. PrA | Page 8 of 9
Preliminary Data Sheet APPLICATIONS
DRIVING CAPACITIVE LOADS
Because of the large bandwidth of the AD8337, when driving capacitive loads, there may be excessive peaking beyond 100 MHz. This peaking can be mitigated by using a series resistor at the output, RSNUB, to isolate the cap load. The only disadvantage of this method is the attenuation introduced by the attenuator that is formed between RSNUB and RLOAD, where the attenuation factor is RLOAD/( RSNUB + RLOAD). The preamplifier also is sensitive to cap loads; however, this is typically not an issue since only the gain setting resistors load it. If it is desired to drive a capacitive load directly from the preamplifier, then it is recommended that the user also inserts a small series resistor between this cap load and the preamp output.
AD8337
BOARD LAYOUT
Because the AD8337 is a high frequency device board layout is critical. In particular it is important to have good ground plane connection to the VCOM pin. Also the ground for the VGA output and should be separated from the ground for the preamp gain setting resistors and the VCOM pin. Coupling through the ground plane from the output to the input can cause peaking at higher frequencies. For layout reasons it helps to visualize how and where the load currents are flowing, this way one can see potential interaction between output and input.
Rev. PrA | Page 9 of 9
PR05575-0-5/05(PrA)

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