AD7760BCP datasheet(15/22 Pages) AD

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Preliminary Technical Data

AD7760

Rev. PrN | Page 15 of 22

DRIVING THE AD7760

The AD7760 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 full power mode, the

differential amplifier should be configured as a first order anti-

alias filter as shown in Figure 16. 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

VIN-

VIN+

Figure 16. Differential Amplifier Configuration

Table 7. Full Power Component Values

ODR

VREF

RIN

RFB

CFB

2.5MHz

4.096v

1kΩ

655 Ω

5.6pF

33pF

2.5MHz

2.5v

TBD Ω

TBD

TBD pF

48kHz

4.096v

TBD Ω

TBD

TBD pF

48kHz

2.5v

TBD Ω

TBD

TBD pF

Figure 17 shows the signal conditioning that occurs using the

circuit in Figure 16 with a ±2.5v input signal biased around

ground using the component values and conditions in the first

row of 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.

+2.5V

-2.5V

+2.5V

-2.5V

+2.048V

0.410V

+3.685V

VIN+

VIN-

+2.048V

0.410V

+3.685V

Figure 17. Differential Amplifier Signal Conditioning

To obtain maximum performance from the AD7760, it is

advisable to drive the ADC with differential signals. However, it

is possible to drive the AD7760 with a single ended signal once

the common mode of the signal is within the range of +0.7V to

+2.1V with VDD3 = 5V or +0.7 to +1.25V with VDD3 = 3.3V. In

this case the on-chip differential amplifier can be used to

convert the signal from single-ended to differential before being

fed into the modulator inputs. Figure 18 shows how a bipolar

single-ended signal biased around ground can be used to drive

the AD7760 with the use of an external op-amp such as the

AD8021.

RIN

RFB

CFB

VIN-

VIN+

AD8021

VIN

Figure 18. Single Ended to Differential Conversion

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