AD8227BRZ-R71 datasheet(23/25 Pages) AD

23 / 25 page

AD8227

Rev. 0 | Page 22 of 2

APPLICATIONS INFORMATION

DIFFERENTIAL DRIVE

Figure 64 shows how to configure the AD8227 for differential

output.

+IN

–IN

REF

AD8227

VBIAS

–

OP AMP

+OUT

–OUT

RECOMMENDED OP AMPS: AD8515, AD8641, AD820.

RECOMMENDED R VALUES: 5kΩ to 20kΩ.

Figure 64. Differential Output Using an Op Amp

The differential output is set by the following equation:

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

The common-mode output 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 AD8227 and not on the op amp or the resistors.

This circuit takes advantage of the AD8227’s precise control of

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

performance and resistor matching 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.

Tips for Best Differential Output Performance

For best ac performance, an op amp with at least 2 MHz gain

bandwidth and 1 V/μs slew rate is recommended. Good choices

for op amps are the AD8641, AD8515, or AD820.

Keep trace lengths from resistors to the inverting terminal of

the op amp as short as possible. Excessive capacitance at this

node can cause the circuit to be unstable. If capacitance cannot

be avoided, use lower value resistors.

PRECISION STRAIN GAGE

The low offset and high CMRR over frequency of the AD8227

make it an excellent choice for bridge measurements. The

bridge can be connected directly to the inputs of the amplifier

(see Figure 65).

2.5V

10µF

0.1µF

AD8227

+IN

–IN

350Ω

–

Figure 65. Precision Strain Gage

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