AD7767 datasheet(20/24 Pages) AD

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AD7767

Rev. 0 | Page 20 of 24

DRIVING THE AD7767

The AD7767 must be driven with fully differential inputs. The

common-mode voltage of the differential inputs to the AD7767

device and thus the limits on the differential inputs are set by

the reference voltage VREF applied to the device. The common-

mode voltage of the AD7767 is VREF/2. Where the AD7767 VREF+

pin is supplied with a 5 V supply (the ADR445 or ADR425), the

common mode is at 2.5 V. This means that the maximum inputs

that can be applied on the AD7767 differential inputs are a

5 V p-p input around 2.5 V.

VIN+

VIN–

VREF

Figure 39. Maximum Differential Inputs to the AD7767

An analog voltage of 2.5 V supplies the AD7767 AVDD pin.

However, the AD7767 allows the user to apply a reference

voltage of up to 5 V. This provides the user with an increased

full-scale range, offering the user the option of using the

AD7767 with a larger LSB voltage size. Figure 39 shows the

maximum and minimum inputs to the AD7767.

DIFFERENTIAL SIGNAL SOURCE

An example of some recommended driving circuitry that can

be employed in conjunction with the AD7767/AD7767-1/

AD7767-2 is shown in Figure 40. Figure 40 shows how the

ADA4841-1 device can be used to drive an input to the AD7767/

AD7767-1/AD7767-2 from a differential source. Each of the

differential paths is driven by an ADA4841-1 device.

ADA4841-1

AIN+

499Ω

3.3Ω

499Ω

3.3nF

ADA4841-1

AIN–

499Ω

1kΩ

3.3Ω

1kΩ

499Ω

3.3nF

AVDD

VIN+

VIN–

VREF+

ADR425

AD7767

2.5V

2× VCM

10nF

Figure 40. Driving the AD7767 from a Fully Differential Source

SINGLE-ENDED SIGNAL SOURCE

In the case where the AD7767 is being supplied from a single-

ended source, the application circuit in Figure 41 can be used to

drive the AD7767 device. Figure 41 shows how the ADA4941-1

single-to-differential amplifier can be used to create a fully

differential input to the AD7767. The single-ended signal input

is applied to the positive input of the ADA4941-1 device.

Arrange the values of the resistor elements to create a 2.5 V

common-mode input to the AD7767/AD7767-1/AD7767-2

device. R3 and C2 default to 3.3 Ω and 10 nF, respectively.

AVDD

VIN+

VIN–

VREF+

ADR425

AD7767

2.5V

VSS–

VSS+

AIN+

VOFFSET

2× VCM

RFB

CFB

ADA4941-1

Figure 41. Driving the AD7767 from a Single-Ended Source

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