AD7767 datasheet(16/24 Pages) AD

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AD7767

Rev. 0 | Page 16 of 24

–20

–40

–60

–80

–100

–120

–140

–160

128k

112k

96k

80k

64k

48k

32k

16k

FREQUENCY (Hz)

Figure 32. AD7767 Digital Filter Frequency Response

–20

–40

–60

–80

–100

–120

–140

–160

56k

48k

40k

32k

24k

16k

FREQUENCY (Hz)

64k

Figure 33. AD7767-1 Digital Filter Frequency Response

–20

–40

–60

–80

–100

–120

–140

–160

28k

24k

20k

16k

12k

FREQUENCY (Hz)

32k

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 C2

VIN–

VREF+

GND

AGND

RIN C2

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. The diodes become

forward biased if the input voltage exceeds this limit 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 connection 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, 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 5 V reference applied to the VREF+ pin.

The recommended reference devices are the ADR445 or the

ADR425. The 5 V reference operates both as a reference supply

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

device. This feature means that 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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