AD623 datasheet(25/28 Pages) AD

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AD8426

Rev. 0 | Page 25 of 28

APPLICATIONS INFORMATION

PRECISION STRAIN GAGE

The low offset and high CMRR over frequency of the AD8426

make it an excellent candidate for bridge measurements. The

bridge can be connected directly to the inputs of the amplifier

(see Figure 68).

2.5V

10µF

0.1µF

AD8426

+IN

–IN

350Ω

–

Figure 68. Precision Strain Gage

DIFFERENTIAL DRIVE

The differential output configuration of the AD8426 has the

same excellent dc precision specifications as the single-ended

output configuration.

Differential Output Using Both AD8426 Amplifiers

The circuit configuration is shown in Figure 69. The differential

output specifications in Table 2, Table 4, Table 5, and Table 7

refer to this configuration only. The circuit includes an RC filter

that maintains the stability of the loop.

+IN1

–IN1

AD8426

–

AD8426

–

100pF

+INx

VOUT–

VOUT+

10kΩ

REF2

Figure 69. Differential Circuit Schematic

The differential output voltage is set by the following equation:

VDIFF_OUT = VOUT+ − VOUT− = G × (VIN+ − VIN−)

where:

The common-mode output voltage is set by the average of +IN2

and REF2. The transfer function is

VCM_OUT = (VOUT+ + VOUT−)/2 = (V+IN2 + VREF2)/2

A common application sets the common-mode output voltage

to the midscale of a differential ADC. In this case, the ADC

reference voltage is sent to the +IN2 terminal, and ground is

connected to the REF2 terminal. This produces a common-

mode output voltage of half the ADC reference voltage.

2-Channel Differential Output Using a Dual Op Amp

Another differential output topology is shown in Figure 70.

Instead of a second in-amp, one-half of a dual op amp creates

the inverted output. The recommended dual op amps (the

AD8642 and the AD822) are packaged in an MSOP. This

configuration allows the creation of a dual-channel, precision

differential output in-amp with little board area.

Figure 70 shows how to configure the AD8426 for differential

output.

+IN

–IN

REF

AD8426

VBIAS

–

OP AMP

VOUT+

VOUT–

RECOMMENDED OP AMPS: AD8642, AD822.

RECOMMENDED R VALUES: 5kΩ TO 20kΩ.

Figure 70. Differential Output Using an Op Amp

The differential output voltage is set by the following equation:

VDIFF_OUT = VOUT+ − VOUT− = G × (VIN+ − VIN−)

where:

The common-mode output voltage is set by the following

equation:

VCM_OUT = (VOUT+ − VOUT−)/2 = VBIAS

The advantage of this circuit is that the dc differential accuracy

depends on the AD8426 and not on the op amp or the resistors.

This circuit takes advantage of the precise control of the AD8426

over its output voltage relative to the reference voltage. Op amp

dc performance and resistor matching do affect the dc common-

mode output accuracy. However, because common-mode errors

are likely to be rejected by the next device in the signal chain, these

errors typically have little effect on overall system accuracy.

For best ac performance, an op amp with gain bandwidth of at

least 2 MHz and a slew rate of at least 1 V/μs is recommended.

Good choices for op amps are the AD8642 and the AD822.

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