AD8228ARMZ-R71 datasheet(20/25 Pages) AD

20 / 25 page

AD8228

Rev. 0 | Page 19 of 24

APPLICATIONS INFORMATION

DIFFERENTIAL DRIVE

Figure 48 shows how to configure the AD8228 for differential

output. The advantage of this circuit is that the dc differential

accuracy depends on the AD8228 and not on the op amp or the

resistors. This circuit takes advantage of the precise control the

AD8228 has of its output voltage relative to the reference voltage.

The ideal equation for the differential output is as follows:

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

Op amp dc performance and resistor matching determine 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. The ideal equation for the common-mode

output is as follows:

VCM_OUT =

−

+ +

OUT

= VREF

For best ac performance, an op amp with at least 3 MHz gain

bandwidth product and 2 V/μs slew rate is recommended.

+IN

–IN

REF

AD8228

VREF

10kΩ

–

AD8641

+OUT

–OUT

10kΩ

Figure 48. Differential Output Using an Op Amp

PRECISION STRAIN GAGE

The low offset and high CMRR over frequency of the AD8228

make it an excellent candidate for bridge measurements. As shown

in Figure 49, the bridge can be connected directly to the inputs

of the amplifier.

2.5V

10µF

0.1µF

AD8228

+IN

–IN

350Ω

–

Figure 49. Precision Strain Gage

DRIVING A DIFFERENTIAL ADC

Figure 50 shows how the AD8228 can be used to drive a

differential ADC. The AD8228 is configured with an op amp and

two resistors for differential drive. The 510 Ω resistors and 2200

pF capacitors isolate the instrumentation amplifier from the

switching transients produced by the switched capacitor front

end of a typical SAR converter. These components between the

ADC and the amplifier also create a filter at 142 kHz, which

provides antialiasing and noise filtering. The advantage of this

configuration is that it uses less power than a dedicated ADC

driver: the AD8641 typically consumes 200 μA, and the current

through the two 10 kΩ resistors is 250 μA at full output voltage.

With the AD7688, this configuration gives excellent dc perform-

ance and a THD of 71 dB (10 kHz input). For applications that

need better distortion performance, a dedicated ADC driver, such

as the ADA4941-1 or ADA4922-1, is recommended.

IN+

VDD

REF

GND

AD7688

IN–

0.1µF

+5V

ADR435

GND

VIN

VOUT

0.1µF

+8V

0.1µF

10µF

X5R

10kΩ

0.1µF

510Ω

0.1µF

510Ω

0.1µF

AD8228

+IN

–IN

REF

+8V

–8V

0.1µF

–8V

+8V

AD8641

10kΩ

Figure 50. Driving a Differential ADC

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