REF19X datasheet(24/28 Pages) AD

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REF19x Series

Rev. I | Page 24 of 28

For VS levels lower than the lower threshold, U3 output is low;

thus, U2 and Q1 are off while U1 is on. For VS levels higher

than the upper threshold, the situation reverses, with U1 off and

both U2 and Q1 on. In the interest of battery power

conservation, all of the comparison switching circuitry is

powered from VS and is arranged so that when VS fails, the

default output comes from U1.

For the R1 to R3 values, as shown in Figure 30, lower/upper VS

switching thresholds are approximately 5.5 V and 6 V, respectively.

These can be changed to suit other VS supplies, as can the REF19x

devices used for U1 and U2, over a range of 2.5 V to5 V of output.

U3 can operate down to a VS of 3.3 V, which is generally

compatible with all REF19x family devices.

LOW POWER, STRAIN GAGE CIRCUIT

As shown in Figure 31, the REF19x family of references can be

used in conjunction with low supply voltage operational amplifiers,

such as the OP492 or the OP283, in a self-contained strain gage

circuit in which the REF195 is used as the core. Other

references can be easily accommodated by changing circuit

element values. The references play a dual role, first as the

voltage regulator to provide the supply voltage requirements of

the strain gage and the operational amplifiers, and second as

a precision voltage reference for the current source used to

stimulate the bridge. A distinct feature of the circuit is that it

can be remotely controlled on or off by digital means via

the SLEEP pin.

500Ω

0.1%

57kΩ

0.1µF

10kΩ

0.1µF

1/4

OP492

10µF

REF195

2.21kΩ

20kΩ

10kΩ

20kΩ

0.01µF

10µF

100Ω

OUTPUT

2N2222

1µF

–

1/4

OP492

–

1/4

OP492

–

1/4

OP492

–

10kΩ

Figure 31. A Low Power, Strain Gage Circuit

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