Data Sheet

AD7767

DRIVING THE AD7767

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Rev. D | 20 of 23

The AD7767 must be driven with fully differential inputs. The com-

mon-mode voltage of the differential inputs to the AD7767 device

and therefore 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. When the AD7767 VREF+ pin has

a 5 V supply (using ADR445, ADR435, or ADR425), the common

mode is at 2.5 V, meaning that the maximum inputs that can be

applied on the AD7767 differential inputs are a 5 V p-p input around

2.5 V.

Figure 39. Maximum Differential Inputs to the AD7767

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

er, 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 greater LSB

voltage. Figure 39 shows the maximum inputs to the AD7767.

DIFFERENTIAL SIGNAL SOURCE

An example of recommended driving circuitry that can be used

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.

SINGLE-ENDED SIGNAL SOURCE

For applications using a single-ended analog signal, either bipolar

or unipolar, the ADA4941-1 single-ended-to-differential driver cre-

ates a fully differential input to the AD7767/AD7767-1/ AD7767-2.

The schematic is shown in Figure 41.

R1 and R2 set the attenuation ratio between the input range and

the ADC range (VREF). R1, R2, and CF are chosen depending on

the desired input resistance, signal bandwidth, antialiasing, and

noise contribution. The ratio of R2 to R1 should be equal to the ratio

of REF to the peak-to-peak input voltage. For example, for the ±10

V range with a 4 kΩ impedance, R2 = 1 kΩ and R1 = 4 kΩ.

R3 and R4 set the common mode on the IN− input, and R5 and R6

set the common mode on the IN+ input of the ADC. The common

mode, which is equal to the voltage present at VOFFSET1, should be

close to VREF/2. The voltage present should roughly be set to the

ratio of VOFFSET1 to 1 + R2/R1.

Figure 40. Driving the AD7767 from a Fully Differential Source

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

Table 8. Resistor Values Required When Using the Differential to Single-Ended Circuit with ADA4941 (See Figure 41)

VIN (V)

VOFFSET1 (V)

VOFFSET2 (V)

OUT+ (V)

OUT− (V)

R1 (kΩ)

R2 (kΩ)

R4 (kΩ)

R3 = R5 = R6 (kΩ)

+20, −20

2.5

2.203

−0.01, +4.96

5.01, 0.04

8.06

1

12.7

10

+10, −10

2.5

2.000

0.01, 4.99

4.99, 0.01

4.02

1

15

10

+5, −5

2.5

1.667

0.00, 5.00

5.00, 0.00

2

1

20

10