AD7765 datasheet(17/32 Pages) AD

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AD7765

Rev. 0 | Page 17 of 32

The common-mode input at each of the differential amplifier

inputs (Pin VINA and Pin VINA−) can range from−0.5 V dc to

2.2 V dc. The amplifier has a constant output common-mode

voltage of 2.048 V, that is, VREF/2, the requisite common-mode

voltage for the modulator input pins (VIN+ and VIN−).

ON-CHIP DIFFERENTIAL AMPLIFIER

The AD7765 contains an on-board differential amplifier that

is recommended to drive the modulator input pins. Pin 1, Pin 2,

Pin 3, and Pin 4 on the AD7765 are the differential input and

output pins of the amplifier. The external components, RIN, RFB,

CFB, CS, and RM, are placed around Pin 1 through Pin 6 to create

the recommended configuration. To achieve the specified

performance, the differential amplifier should be configured as

a first-order antialias filter, as shown in

Figure 29 shows the signal conditioning that occurs using the

differential amplifier configuration detailed in Table 7 with a

±2.5 V input signal to the differential amplifier. The amplifier

in this example is biased around ground and is scaled to give

±3.168 V p-p (−0.5 dBFS) on each modulator input with a

2.048 V common mode.

Figure 28 using the

component values listed in Table 7. The inputs to the

differential amplifier are then routed through this external

component network before being applied to the modulator

inputs VIN− and VIN+(Pin 5 and Pin 6). Using the optimal

values in the table as an example yields a 25 dB attenuation at

the first alias point of 19.84 MHz.

+2.5V

–2.5V

+2.5V

–2.5V

+3.632V

+2.048V

+0.464V

+3.632V

+2.048V

+0.464V

VIN+

VIN–

DIFF

AMP

RIN

RFB

CFB

RIN

RFB

CFB

VIN–

VIN+

VINA–

VINA+

VOUTA+

VOUTA–

Figure 29. Differential Amplifier Signal Conditioning

To obtain maximum performance from the AD7765, it is advisable

to drive the ADC with differential signals.

Figure 28. Differential Amplifier Configuration

Figure 30 shows how a

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

AD7765 with the use of an external op amp, such as the AD8021.

Table 7. On-Chip Differential Filter Component Values

RIN

(kΩ)

RFB

(kΩ)

(Ω)

(pF)

CFB

(pF)

DIFF

AMP

RIN

RFB

CFB

RIN

RFB

CFB

VIN–

VIN

VIN+

AD8021

Optimal

4.75

3.01

8.2

Tolerance

Range1

2.37 to

5.76

2.4 to

4.87

36 to

0 to 10

20 to

100

39 to

1 Values shown were the acceptable tolerances for each component when

altered relative to the optimal values used to achieve the stated

specifications of the device.

The range of values that can be used for each of the listed

components in the differential amplifier configuration is also

listed in Table 7. When using the differential amplifier to gain

the input voltages to the required modulator input range, it is

advisable to implement the gain function by changing RIN,

leaving the RFB as the listed optimal value.

Figure 30. Single-Ended-to-Differential Conversion

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