AD7764 datasheet(17/21 Pages) AD

17 / 21 page

Preliminary Technical Data

AD7764

Rev. PrC | Page 17 of 21

DRIVING THE AD7764

The AD7764 has an on-chip differential amplifier. This

amplifier will operate with a supply voltage (AVDD3) from 3V to

5.5V. For a 4.096V reference, the supply voltage must be 5V.

To achieve the specified performance in normal power mode,

the differential amplifier should be configured as a first order

anti-alias filter as shown in Figure 20. 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 listed in

Table 7. Using the first row as an example would yield a 10dB

attenuation at the first alias point of 19MHz.

RIN

RFB

CFB

RIN

RFB

CFB

VIN–

VIN+

Figure 20. Differential Amplifier Configuration

Table 7.First-Order Filter Component Values

VREF

RIN

RFB

CFB

4.096v

4.75kΩ

3.01kΩ

43Ω

1.2pF

33pF

Figure 21 shows the signal conditioning that occurs using the

circuit in Figure 20 with a ±2.5V input signal biased around

ground using the component values and conditions in Table 7.

The differential amplifier will always bias the output signal to sit

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

The signal is also scaled to give the maximum allowable voltage

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

VREF, i.e. 0.8 × 4.096V ≈ 3.275V peak to peak on each input.

To obtain maximum performance from the AD7764, it is advisable

to drive the ADC with differential signals. Figure 22 shows how a

bipolar, single-ended signal biased around ground can drive the

AD7764 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).

+2.5V

–2.5V

+2.5V

–2.5V

+3.685V

+2.048V

+0.410V

+3.685V

+2.048V

+0.410V

VIN+

VIN–

Figure 21. Differential Amplifier Signal Conditioning.

RIN

RFB

CFB

VIN-

VIN+

AD8021

VIN

Figure 22. Single Ended to Differential Conversion

The AD7764 employs a double sampling front end,as shown in

Figure Figure 23. For simplicity, only the equivalent input

circuitry for VIN+ is shown. The equivalent circuitry for VIN- is

the same.

CS2

CPB2

SS4

SH4

CPA

SS2

SH2

CS1

CPB1

SS3

SH3

SS1

SH1

ANALOG

MODULATOR

VIN+

Figure 23. Equivalent Input Circuit

AD7764 Datasheet(PDF) 17 Page - Analog Devices

AD7764 Datasheet(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL