TC500A/TC520 AUTO-RANGING APPLICATION

APPLICATION NOTE 1

TC500A/TC520 AUTO-RANGING APPLICATION By Ted Dabney

Since this application uses the PC's printer port for communications and power, four attenuators are used to reduce the amplitude of the analog switch control signals. Some leakage between the control lines and the signal lines would occur if this wasn't done. The TC520 clock is supplied from a 1.0703MHz crystal. There is nothing special about this frequency except that it just happened to be available. Due to the high sensitivity and the high resolution required, it is imperative that the integration time of the TC500A be set so that all 50Hz and 60Hz noise is eliminated. This is done by pre-loading the TC520 with an 8-bit value which will modify the Auto-Zero and Integrate timing:

One of the TC500KIT prototypes was modified to perform as an Auto-Ranging volt meter. A Caddock precision resistive divider, originally specified for our TC818 ADC, was used as the selectable input attenuator. Two analog switches (TC4202) were used to select one of five input ranges. The requirement for this application was to resolve 10µV with minimum jitter and provide a full 5-digit display ("99999"). 100,000 counts requires a 17-bit conversion but the TC500A is only a 16-bit device so some average was needed. The full-scale value of each of the five ranges are: 999.99 mV 9.9999 V 99.999 V 999.99 V 1999.9 V *

1)

Select a desired time for TINT. TINT = 200 mS Determine the TC520 clock rate. TCLK = 4 ÷ fo = 3.737µS/count Calculate the number of counts required. K = TINT ÷ TCLK = 53518.8 Divide K by 256 (fixed count multiplier) K ÷ 256 = 209 Compliment the results (down-counter) 256 - 209 + 47 or 2FHEX

2) 3)

(Range 5 is limited to 2KV due to dissipation of the 10MΩ input resister.)

4) 5)

to switches

+5V 180K .47µF

9.9M

1.0703 MHz

10K

TC4202 (X2)

VOLTAGE INPUT 4 S1 S2 S3 S4

6

.68µF 3

13

75K

1.11M

7

1 16

1

9

14

3

10

13

4

12

5

8

15

2

12

11

ANALOG GROUND 101K

S1

10K

S2

20K 1.5nF

S3

1K

S4

TC04

S1 S2 S3 S4

9 10 8 5

TC500A

.1µF 2

3.9K

12K

3.9K

12K

3.9K

12K

3.9K

11

7 .68µF

to I/O Port

12K

6

TC520

to I/O Port

1) 2) 3) 4) 5)

GND to switches 10µF 5

10µF 3

8

TC962 4

2 .10µF

Figure 1 AN-1

1

TC500A/TC520 AUTO-RANGING APPLICATION AN-1 The routine used for this application uses Avr = 16 and dV = 4. The advantage of this method is that it uses only one permanent register instead of the 16 that would be needed for a 'first-in, last out' running average technique. The disadvantage is that it responds exponentially. The 'delta' test greatly improves the response time.

When this value (00101111BIN) is loaded into the TC520 through the serial input, the integration time for the TC500A will be as follows: TINT = TCLK x 256 x 209 = 199.96mS (This should give rejection of about 98dB @ 60Hz and 96dB @ 50Hz.) An exponential averaging technique was employed because it is best suited to the limited RAM space of microprocessors and microcontrollers. This scheme is explained in the following manner: 1) 2) 3)

4)

5) 6)

Add value (V) to summing register (SumReg) SumReg = SumReg + V Take a test value (TempV) TempV = SumReg / Avr Test the 'delta' value if V > (TempV + dV) or V < (TempV - dV) then increment count (count = count +1) otherwise count = 0 Test count if count > 3 then SumReg = V x Avr and count = 0 Extract new average value (V) V = SumReg / Avr Normalize the summing register SumReg = SumReg - V

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Printed in the U.S.A. 9/5/95

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