AN780

© 2002 Microchip Technology, Inc.

DS00780A-page 2

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.

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

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

Ω so a single-ended configuration

is more than adequate. Instead of 1.5V, the actual full-scale output

above the 68k

Ω minimum. This gives a maximum buffer current of

7.6

µA instead of 20µA.

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

of 4V will allow for some offset. Rearranging equation 1 gives an

= 1V 200mS/3.5V 130k = .439mF

eq2

The next higher common value is .47

µF which was selected for

very low dielectric absorption.

REFERENCE VOLTAGE CIRCUIT

The differential reference voltage is derived by the standard, dual-

slope ratiometric technique:

eq3

conversion.

INT

BUF

IN+

IN–

REF+

REF–

COM

1

4

3

+V

COMP

B

A

CR–

CR+

GND

–V

+V

LOAD

READ

100k

LM285-2.5

Analog Ground

.01

µ

.01u

11

10

9

8

5

100k

16

COMP

B

A

DV

CE

GND

14

13

12

6

7

15

2

–5V

DGND

2

14

13

5

4

Crystal

6

7

3

12

8

10

11

9

TC520

TC500A

+5V

SI

SO

SK

RD

LD

10k

DV

CE

1

FIGURE 2: TC500A and TC520