AD8224ACPZ-R7 datasheet(24/27 Pages) AD

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AD8224

Preliminary Technical Data

APPLICATIONS

DRIVING AN ANALOG-TO-DIGITAL CONVERTER

+IN

store and deliver necessary charge to the switched capacitor input

of the ADC. The 500 Ω series resistor reduces the burden of the

–

AD8224

REF2

An instrumentation amplifier is often used in front of an analog-to-

+OUT

+IN2

digital converter to provide CMRR and additional conditioning

20kΩ

–IN

such as a voltage level shift and gain (see Figure 63). In this

example, a 2.7 nF capacitor and a 500 Ω resistor create an anti

33pF

aliasing filter for the AD7685. The 2.7 nF capacitor also serves to

2.7 nF load from the amplifier. However, large source impedance in

–OUT

front of the ADC can degrade total harmonic distortion (THD).

Figure 64. Differential Circuit Schematic

For applications where THD performance is critical, the series

Setting the Common-Mode Voltage

resistor needs to be small. At worst, a small series resistor can load

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

the AD8224, potentially causing the output to overshoot or ring.

and REF2. The transfer function is

In such cases, a buffer amplifier, such as the AD8615, should be

used after the AD8224 to drive the ADC.

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

+5V

+IN2 and REF2 have different properties that allow the

AD8224

AD7685

ADR435

+5V

2.7nF

REF

500Ω

1.07kΩ

+2.5V

+IN

–IN

0.1µF

10µF

reference voltage to be easily set for a wide variety of applications.

+IN2 has high impedance but cannot swing to the supply rails

4.7µF

of the part. REF2 must be driven with a low impedance, but can

go 300 mV beyond the supply rails.

±50mV

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.

Figure 63. Driving an ADC in a Low Frequency Application

2-Channel Differential Output Using a Dual Op Amp

Another differential output topology is shown in Figure 65.

Instead of a second in-amp, ½ of a dual OP2177 op amp creates

the inverted output. Because the OP2177 comes in an MSOP,

this configuration allows the creation of a dual channel,

precision differential output in-amp with little board area.

Errors from the op amp are common to both outputs and are

thus common mode. Errors from mismatched resistors also

create a common-mode dc offset. Because these errors are

common mode, they are likely to be rejected by the next device

in the signal chain.

+IN

AD8224

4.99kΩ

–

OP2177

4.99kΩ

+OUT

REF

VREF

DIFFERENTIAL OUTPUT

The differential configuration of the AD8224 has the same

excellent dc precision specifications as the single-ended output

configuration and is recommended for applications in the

frequency range of dc to 100 kHz.

The circuit configuration, outlined in Table 7, refers to the

configuration shown in Figure 64 only. The circuit includes an RC

filter that maintains the stability of the loop.

The transfer function for the differential output is:

VDIFF_OUT

= V+OUT − V−OUT = (V+IN − V−IN) × G

where:

49.4 kΩ

G = 1 +

–IN

–OUT

Figure 65. Differential Output Using Op Amp

Rev. PrB | Page 24 of 27

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