AD7767 datasheet(17/25 Pages) AD

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AD7767

Rev. C | Page 16 of

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128k

112k

96k

80k

64k

48k

32k

16k

FREQUENCY (Hz)

Figure 32. AD7767 Digital Filter Frequency Response

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64k

56k

48k

40k

32k

24k

16k

FREQUENCY (Hz)

Figure 33. AD7767-1 Digital Filter Frequency Response

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–80

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32k

28k

24k

20k

16k

12k

FREQUENCY (Hz)

Figure 34. AD7767-2 Digital Filter Frequency Response

ANALOG INPUT STRUCTURE

The AD7767/AD7767-1/AD7767-2 are configured as a differential

input structure. A true differential signal is sampled between the

analog inputs VIN+ and VIN−, Pin 4 and Pin 5, respectively. Using

differential inputs provides rejection of signals that are common

to both the VIN+ and VIN− pins.

Figure 35 shows the equivalent analog input circuit of the

AD7767/AD7767-1/AD7767-2. The two diodes on each of the

differential inputs provide ESD protection for the analog inputs.

VIN+

VREF+

GND

AGND

RIN

VIN–

VREF+

GND

AGND

RIN

Figure 35. Equivalent Analog Input Structure

Take care to ensure that the analog input signal does not exceed

the reference supply voltage (VREF+) by more than 0.3 V, as specified

in the Absolute Maximum Ratings section. If the input voltage

exceeds this limit, the diodes become forward biased and start

to conduct current. The diodes can handle 130 mA maximum.

The impedance of the analog inputs can be modeled as a parallel

combination of C1 and the network formed by the series con-

nection of RIN, C1, and C2. The value of C1 is dominated by the

pin capacitance. RIN is typically 1.4 kΩ, the lumped component

of serial resistors and the RON of the switches. C2 is typically

22 pF, and its value is dominated by the sampling capacitor.

SUPPLY AND REFERENCE VOLTAGES

The AD7767/AD7767-1/AD7767-2 operate from a 2.5 V supply

applied to the DVDD and AVDD pins. The interface is specified to

operate between 1.7 V and 3.6 V. The AD7767/AD7767-1/

AD7767-2 operate from a reference input in the range of 2.2 V

to 2 × AVDD applied to the VREF+ pin. The nominal reference

supply voltage is 5 V, but a 2.5 V supply can also be used. When

using a 5 V reference, the recommended reference devices are

the ADR445, ADR435, or ADR425; when using 2.5 V, the ADR441,

ADR431, or ADR421 are recommended. The voltage applied to

the reference input (VREF+) operates both as a reference supply

and as a power supply to the AD7767/AD7767-1/AD7767-2

device. Therefore, when using a 5 V reference input, the full-scale

differential input range of the AD7767/AD7767-1/AD7767-2

is 10 V. See the Driving the AD7767 section for details on the

maximum input voltage.

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