AD974 datasheet(13/20 Pages) AD

13 / 20 page

REV. A

AD974

–13–

Table I. Analog Input Configuration

Input Voltage

Connect

Input

Range

VxA to

VxB to

Impedance

±10 V

BIP

VIN

13.7 k

Ω

0 V to +5 V

VIN

GND

6.0 k

Ω

0 V to +4 V

VIN

6.4 k

Ω

Table II. Output Codes and Ideal Input Voltage

Digital Input

Description

Analog Input

Straight Binary

Full-Scale Range

±10 V

0 V to +5 V

0 V to +4 V

Least Significant Bit

305

µV

+Full Scale (FS – 1 LSB)

+9.999695 V

+4.999847 V

+3.999939 V

1111 1111 1111 1111

Midscale

0 V

+2.5 V

+2 V

1000 0000 0000 0000

One LSB Below Midscale

–305

µV

+2.499924 V

+1.999939 V

0111 1111 1111 1111

–Full Scale

–10 V

0 V

0000 0000 0000 0000

ANALOG INPUTS

The AD974 is specified to operate with three full-scale analog

input ranges. Connections required for each of the eight analog

inputs, VxA and VxB and the resulting full-scale ranges, are

shown in Table I. The nominal input impedance for each ana-

log input range is also shown. Table II shows the output codes

for the ideal input voltages of each of the analog input ranges.

The analog input section has a

±25 V overvoltage protection on

VxA and VxB. Since the AD974 has two analog grounds it is

important to ensure that the analog input is referenced to the

AGND1 pin, the low current ground. This will minimize any

problems associated with a resistive ground drop. It is also

important to ensure that the analog inputs are driven by a low

impedance source. With its primarily resistive analog input

circuitry, the ADC can be driven by a wide selection of general

purpose amplifiers.

To achieve the low distortion capability of the AD974 care

should be taken in the selection of the drive circuitry

op amp.

Figure 10 shows the simplified analog input section for the

AD974. Since the AD974 can operate with an internal or exter-

nal reference, and three different analog input ranges, the full-

scale analog input range is best represented with a voltage that

spans 0 V to VREF across the 40 pF sampling capacitor. The on-

chip resistors are laser trimmed to ratio match for adjustment of

offset and full-scale error using fixed external resistors.

BIP

AGND1

REF

CAP

VxA

VxB

AGND2

12k

SWITCHED

CAP ADC

2.5V

REFERENCE

40pF

AD974

Figure 10. Simplified Analog Input

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