DS00780A-page 3

© 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 30

°C range. This could be a prohibitive

requirement. Fortunately the strain gauge has an output sensitivity

which is directly proportional to the supply voltage applied,

eq3/a

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:

By deriving the reference from the supply voltage, any variations

will exactly cancel.

eq3/b

(V

(V

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:

eq4

and the gain of the amplifier (G). The actual differential reference

BUF

6

72

9

11

5

41

3

TC500A

Analog

Common

10

8

Ground

–5V

.68

µ

+5V

R1

R2a

R2b

R3

R4

6

4

3

2

RA

7

RB

–

+

V–

V+

+

Strain Gauge

–

+

FIGURE 3: Differential ratiometric reference voltage.