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AN780
15-Kilogram Scale Using the TC500A and the TC520
Author: Ted Dabney, Microchip Technology, Inc. The TC500A has no agenda of its own so it can be used to generate slow, high resolution conversions or fast, low resolution conversions. The trade-off for accuracy is about 1000 counts per millisecond of integration time, i.e., 16-bits with TINT approximately equal to 60mS. Typically, the total conversion time is about 4 times the integration time but, with the TC500A, this is quite flexible. TINT = CINT RINT VINT/VIN (max) eq1
INTRODUCTION
A 15kg weighing scale was designed using Microchip's TC500A Analog Processor and the TC520 16-bit Controller. The scale is required to resolve down to 1/8 gram and correct to within 61/2 gram. This project takes into account all aspects of a functional scale: * * * * * Dynamic Range Strain Gauge Compensation Zeroing Oversampling Units Conversion (kilograms to pounds)
The TC520 is a digital interface device which can be used to replace all of the TC500A timing and counting functions performed by a microprocessor. The TC520 can use either a crystal or external clock as a time-base to control the operation of either a TC500 or a TC500A.
The TC500A is an analog processor device which performs a dualslope analog-to-digital conversion function. All of the counting and timing for the conversion must be controlled by an external source. In nearly all applications, this control source is a microprocessor. The microprocessor is programmed to monitor the status and to control the timing of the TC500A. It must also be programmed to count the conversion results.
CREF + CREF V+ REF 79 + VREF
RINT CAZ 86 C-
REF
CINT
A 0 0 1 1
B 0 1 0 1
Control Logic Converter State Zero Integrator Output Auto-zero Signal Integrate Deintegrate
4
BUF
3
CAZ Integrator - +
1
CINT TC500/TC500A Comp 1 + -
SWR SWR SWI V+ 11 IN
Buffer -
SW-
RI
SW+ RI
+
Comp 2 - +
Level Shift
14 Comp Output
SWZ Analog Common - VIN 5 SWI
SW+ RI
SW-
SWIZ
SWZ Polarity Detection Analog Switch Control Signals
RI
SW1
10
Phase Decoding Logic 12 13
15
GND
2 - VS
16 + VS
A B Control Logic
FIGURE 1: Functional block diagram.
(c) 2002 Microchip Technology, Inc.
DS00780A-page 1
AN780
+5V
*CINT
10k
1 INT 3 CAZ 4 BUF 11 IN+
10 IN- 9 REF+
+V 16
Crystal
6 7 3 4 5 13 14
OSCOUT
OSCIN COMP B A DV CE
CAZ
RINT
.01u
100k
VIN+
VIN-
COMP 14 B 13 A 12 CR- 6 CR+ 7 GND 15 -V 2
CREF
+V LOAD READ DCLK DIN DOUT
1 12 8 10 11 9
LD RD
SK
SO
SI
.01 100k LM285-2.5 Analog Ground
8 REF- 5 COM
TC500A
2 GND
TC520
CE
DGND
*CINT recommended Polypropylene
DV
-5V
FIGURE 2: TC500A and TC520
The 16-bit conversion result is accumulated in the TC520 along with a polarity bit and an overrange bit. These bits are formed into one 18-bit serial word which may be read at any rate and at any time. Reading the serial data from the TC520 does not effect the TC500A/TC520 conversion cycle except that the output shift register will not update while reading is in progress.
Integrator Stage
The signal-to-noise ratio of the TC500A's integrator stage is a function of the band-width. The 15kG scale needs to resolve 1g with at least 8:1 over-sampling. This means at least 120,000 counts. The above rule of "1000 counts per millisecond" requires at least 120ms for the integration time of the TC500A. Selecting 200mS will lower the band-width and get maximum rejection of 50/ 60Hz. The strain gauge is a balanced bridge so the output will have some common mode component. A value of 3.5V for VINT instead of 4V will allow for some offset. Rearranging equation 1 gives an expression for CINT: CINT = VIN (max) TINT/VINT RINT = 1V 200mS/3.5V 130k = .439mF
DEVELOPING THE SCALE APPLICATION USING THE TC500A AND THE TC520
Input Stage
The first consideration for a low signal level source is the amount of gain required for the input amplifier. The TC500A has a CMOS input buffer which, due to unity-gain phase margin, must have no lower than about 68k for RINT. The maximum buffer current VIN(max)/RINT) should be no more than about 20A. This means that the maximum input voltage to the TC500A should be about 1.5V. The 15kG strain gauge used for this application has an output of about 1mV/gram which gives a gain requirement of at least 50. The MCP606 CMOS operational amplifier is best suited for this because of its low noise and minimal drift. The output impedance of the strain gauge is only 300 so a single-ended configuration is more than adequate. Instead of 1.5V, the actual full-scale output wound up to be about 1V. The value of RINT was set to 130k, well above the 68k minimum. This gives a maximum buffer current of 7.6A instead of 20A.
eq2
The next higher common value is .47F which was selected for CINT. It is essential that this capacitor is a polypropylene type for very low dielectric absorption.
REFERENCE VOLTAGE CIRCUIT
The differential reference voltage is derived by the standard, dualslope ratiometric technique: VREF = VIN (max) TINT/TDEINT eq3
where TDEINT is the deintegration time required for a full-scale conversion.
DS00780A-page 2
(c) 2002 Microchip Technology, Inc.
AN780
This application requires 120,000 counts which means that the TC520's overrange bit must be used as the MSB, i.e., 17-bits. A reference voltage with a tempco of 0.3ppm/C would normally be required for stability over a 30C range. This could be a prohibitive requirement. Fortunately the strain gauge has an output sensitivity which is directly proportional to the supply voltage applied, VSG = K (V+ - V-)PSG, eq3/a Notice that VIN has been replaced by an expression for the pressure on the strain gauge (PSG), the strain gage constant (K) and the gain of the amplifier (G). The actual differential reference voltage is determined only by the ratio of resistance values (RTOTAL/ RREF ). The TC500A has a differential reference input so the reference voltage need not be referenced to ground. Rather than using a precision reference for the TC500A and a precision supply for the strain gauge, combining eq3/a and eq3/b into eq4 produces an equation for the system: TDEINT = K G PSG TINT RTOTAL/RREF eq4
where K is the constant for a particular strain gauge and the dual slope converter produces a result which is inversely proportional to its reference voltage: TDEINT = VIN TINT/VREF By deriving the reference from the supply voltage, any variations will exactly cancel. VREF = RREF x (V+ - V-)/RTOTAL eq3/b
+5V
RINT
CINT CAZ
V+
R1
4 BUF 11
3
CAZ
1 CINT
RB
PSG Strain Gauge
-
VIN+
VIN- TC500A
2
RA
7 -
10
6
+
VSG
3+ 4
VIN
+
R2a R2b R3
9
VREF+
Analog CREF+ CREF- V- Common 6 7 2 5
.68 Ground -5V
8V REF-
VREF
R4
V-
-VSG = K PSG (V+- V-) G = -RB/RA VIN = (V+- V-) PSG K G
RREF = R2B + R3 RTOTAL = R1 + R2 + R3 + R4 VREF = (V+- V-) RREF/RTOTAL
TDEINT =
VIN VREF
TINT =
(V+- V-) PSG K G (V+- V-) RREF/RTOTAL
TINT = PSG K G
RTOTAL RREF
TINT
FIGURE 3: Differential ratiometric reference voltage.
(c) 2002 Microchip Technology, Inc.
DS00780A-page 3
AN780
AUTO-ZERO AND REFERENCE CAPACITORS
The voltage on these capacitors stay very constant so dielectric absorption is not a consideration. The long integration time does require capacitors with very low leakage. A .68F polyester capacitor was used in both cases. There are 4 clocks/count in the TC520 and the base integration counter is 256 counts. This calculates to a timebase period of 0.9567mS with the crystal being used. The 200mS integration time requires 209 timebase periods. Since the TC520 gives 256 timebase periods, 47 of them need to be taken away. The value can be determined from the equation: N = 256 - fOSC x TINT = 256 - 1.0703MHz x 200mS = 46.957 1024 1024
TC520 TIMING
A 200ms integration time is already selected. There are a few options available with the TC520 to do this. The exact crystal (or clock rate) can be select in conjunction with one of the two default timings in the TC520 or, the microprocessor can be used to program the TC520 for the proper timing with some arbitrary crystal frequency. The main constraint is that the TC500A has a comparator delay of about 4S. Also, the TC520 has a divide-by-4 on the clock input. This means that anything around 1MHz will be acceptable. The TC520 can be programmed by the micro to set the actual integration time to within approximately .5ms. The crystal used in this application is 1.0703MHz.
The micro was programmed to load a "47" (2FH) into the TC520 at the start of the program. This will cause the TC500A to have an integration time of 199.96mS. This value will give at least 120dB of rejection at 50/60Hz. The TC520 will also use the integration timing for the TC500A's Auto-Zero phase. A 17-bit conversion will require a deintegration time which is a function of the oscillator frequency, i.e., 217 x 4 / fOSC = 490mS.
DS00780A-page 4
(c) 2002 Microchip Technology, Inc.
(c) 2002 Microchip Technology, Inc.
TC7211AM
V+ GND DS1 DS2 CS1 CS2 B0 B1 B2 B3 OSC
V+ GND
DS1 DS2 CS1 CS2 B0 B1 B2 B3 OSC
FIGURE 4: Kilogram scale schematic.
TC7211AM
+5V
Reset
47K
20
130k
1.07MHz
1 Reset
.47
MCP606
IN+ 33K
20
0.1
22K
11
4 BUF
.68 3 1 CAZ CINT
7 6 OSCIN OSCOUT
VCC 28 RB7
RB6 27
RB5 26
18 16 14
2 4 6
47K x 8
1.0M
VIN+
VIN-
COMP 14
LOAD
RC1 RC5/SO RC3/SK 15 RC4/SI 17 RC6 18 RC1
GND 8, 19
V+ 16
RC0
1 V+ 3 COMP
12
12 16 14
DV 13
11
-
2
10
7 -
TC520
TC500A
15K Strain Gauge
20K
.5V
6
+
47
15K 0V
0.1 9
IN- 33K
B 13 A 12
DCLK DOUT 9 READ 8
0.1
4B 5A 2 DGND
10 DIN 11
RB4 RB3
25 24 RB2 23 RB1 22 RB0
CKI 9
12 9 7 5
8 11 13 15
PIC16C62A
74HTTC244
+
3+ 4
10K .3V
VREF+
DGND 15
8V REF-
Analog CREF+ CREF- V- Common 7 2 6 5
24K
21
3
1, 19
17
.68
GND
8MHz
-5V
5V Power Supply
8MHz OSC. (HC04)
AN780
DS00780A-page 5
AN780
READ Read Format DOUT OUT DCLK
OVR POL MSB LSB
LOAD Load Format DIN DCLK
MSB LSB
FIGURE 5: Serial interface protocol.
REFERENCE VOLTAGE CALCULATION
Now that the timing has been determined, eq3 can be used to calculate the reference voltage: VREF = VIN (max) TINT/TDCINT = 1V 200mS/490mS .408V The reference voltage does not need to be calculated very precise since it will have to be trimmed during calibration. A 25% adjustment range is enough to make up for just about any minor calculation error.
CONCLUSION
The scale works extremely well. The 8X oversampling makes it very smooth and noise-free. The response time is within one conversion (1/2 sec) for changes of 2 grams or more. Changes of less than 2 grams are accumulated in an integrating register until it gets to either +1 gram or -1 gram. When this happens, the current conversion is allowed to "get through" and a new base is established in the accumulator. There is also a facility in the programming that allows the raw data to be displayed. These displays show the full 17-bit conversion results. The basic converter noise is as predicted, typically 1 to 2 counts of flicker (16-bit accuracy) with an intermittent jump of about 3 or 4 counts (1/f noise). One count is equivalent to 1/8 gram. The actually 60Hz power line rejection ration of the TC500A was not measured, but judging from the 6 to 8 counts of "rolling noise" before preloading the TC520 with 2FH, it is quite adequate. The effect of the differential ratiometric reference was tested by changing the supply voltage from +4V to +6V. Although there was a 1 - 2 second delay due to unmatched time constants between the reference and the strain gauge, the final readings were exactly the same. This shows that the power supply rejection is better than 100dB.
MICROPROCESSOR PROGRAMMING
The PIC16C62A 8-Bit microcontroller was selected but any reasonable processor/controller will suffice. The PIC16C62A is a 28-pin part that has EPROM programmability.
DS00780A-page 6
(c) 2002 Microchip Technology, Inc.
AN780
Start
Power
Setup
Output A=0 B=0
Zero Integrate
No Conversion Complete ? Yes Read Conversion Results from TC520
Comparator ? High Output A=0 B=1 Start Timer
Low
Auto Zero
Save Results as Offset Value
15 Times ? Yes
No
Clear Count Overrrange
Timer Overflow ? Yes Output A=1 B=0 Start Timer
No
No Conversion Complete ? Yes
Latch Count Overrange Polarity
Integrate
Read Conversion Results from TC520
Subtract Offset Value
Stop Counter Timer Overflow ? Yes High Set Polarity Bit Comparator ? No
Add Delta to Register
Delta = This Reading - Saved Reading
No
Low Reset Polarity Bit
Register Overflow ? Yes
Delta > 2 Grams ? Yes Restore Register
Save This Reading
Output A=1 B=1 Start Counter
Deintegrate
Retrieve Saved Reading Divide by 8
Yes
Pounds ? No Convert Binary to BCD
Low
Comparator ? High Set Overrange Bit Yes
Multiply by 22046
No
Counter Overflow ?
Display
FIGURE 6: TC520 program flow chart.
FIGURE 7: PIC16C62A program flow chart.
(c) 2002 Microchip Technology, Inc.
DS00780A-page 7
AN780
NOTES:
DS00780A-page 8
2002 Microchip Technology Inc.
Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip's products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights.
Trademarks The Microchip name and logo, the Microchip logo, FilterLab, KEELOQ, microID, MPLAB, PIC, PICmicro, PICMASTER, PICSTART, PRO MATE, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. dsPIC, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, MXDEV, PICC, PICDEM, PICDEM.net, rfPIC, Select Mode and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. Serialized Quick Turn Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. (c) 2002, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999. The Company's quality system processes and procedures are QS-9000 compliant for its PICmicro(R) 8-bit MCUs, KEELOQ(R) code hopping devices, Serial EEPROMs and microperipheral products. In addition, Microchip's quality system for the design and manufacture of development systems is ISO 9001 certified.
2002 Microchip Technology Inc.
DS00780A - page 9
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DS00780A
DS00780A-page 10
2002 Microchip Technology Inc.

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