AD7763 datasheet(20/32 Pages) AD

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AD7763

Rev. 0 | Page 20 of 32

DRIVING THE AD7763

The AD7763 has an on-chip differential amplifier that operates

with a supply voltage (AVDD3) from 3.15 V to 5.25 V. For a 4.096 V

reference, the supply voltage must be 5 V.

To achieve the specified performance in normal mode, the

differential amplifier should be configured as a first-order

antialias filter, as shown in Figure 31. Any additional filtering

should be carried out in previous stages using low noise, high

performance op amps, such as the AD8021.

Suitable component values for the first-order filter are shown in

Table 10. The values in Table 10 yield a 10 dB attenuation at the

first alias point of 19 MHz.

RIN

RFB

CFB

RIN

RFB

CFB

VIN–

VIN+

Figure 31. Differential Amplifier Configuration

Table 10. Normal Mode Component Values

VREF

RIN

RFB

CFB

4.096 V

1 kΩ

655 Ω

18 Ω

5.6 pF

33 pF

Figure 32 shows the signal conditioning that occurs using the

circuit in Figure 18 with a ±2.5 V input signal biased around

ground and having the component values and conditions in

Table 10.

The differential amplifier always biases the output signal to sit

on the optimum common mode of VREF/2, in this case, 2.048 V.

The signal is also scaled to give the maximum allowable voltage

swing with this reference value. This is calculated as 80% of

VREF; that is, 0.8 × 4.096 V ≈ 3.275 V p-p on each input.

To obtain maximum performance from the AD7763, it is

advisable to drive the ADC with differential signals. Figure 33

shows how a bipolar, single-ended signal biased around ground

can drive the AD7763 with the use of an external op amp, such as

the AD8021.

With a 4.096 V reference, a 5 V supply must be provided to the

reference buffer (AVDD4). With a 2.5 V reference, a 3.3 V supply

must be provided to AVDD4.

+2.5V

–2.5V

+2.5V

–2.5V

+3.685V

+2.048V

+0.410V

+3.685V

+2.048V

+0.410V

VIN+

VIN–

Figure 32. Differential Amplifier Signal Conditioning

RIN

RFB

CFB

RIN

RFB

CFB

VIN–

VIN

VIN+

AD8021

Figure 33. Single-Ended-to-Differential Conversion

CS2

CPB2

SS4

SH4

CPA

SS2

SH2

CS1

CPB1

SS3

SH3

SS1

SH1

ANALOG

MODULATOR

VIN+

Figure 34. Equivalent Input Circuit

The AD7763 employs a double sampling front end, as shown in

Figure 34. For simplicity, only the equivalent input circuit for

VIN+ is shown. The equivalent input circuitry for VIN− is the same.

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