AD977BRZ datasheet(14/24 Pages) AD

14 / 24 page

AD977/AD977A

–14–

REV. D

Table I. AD977A Analog Input Configuration

Input Voltage

Connect R1IN

Connect R2IN

Connect R3IN

Input

Range

via 200

via 100

Impedance

±10 V

VIN

AGND

2.5 V

11.5 k

Ω

±5 V

AGND

VIN

2.5 V

6.7 k

Ω

±3.3 V

VIN

2.5 V

5.4 k

Ω

0 V to 10 V

AGND

VIN

AGND

6.7 k

Ω

0 V to 5 V

AGND

VIN

5.0 k

Ω

0 V to 4 V

VIN

AGND

VIN

5.4 k

Ω

Table II. AD977 Analog Input Configuration

Input Voltage

Connect R1IN

Connect R2IN

Connect R3IN

Input

Range

via 200

via 100

Impedance

±10 V

VIN

AGND

CAP

22.9 k

Ω

±5 V

AGND

VIN

CAP

13.3 k

Ω

±3.3 V

VIN

CAP

10.7 k

Ω

0 V to 10 V

AGND

VIN

AGND

13.3 k

Ω

0 V to 5 V

AGND

VIN

10.0 k

Ω

0 V to 4 V

VIN

AGND

VIN

10.7 k

Ω

ANALOG INPUTS

The AD977/AD977A is specified to operate with six full-scale

analog input ranges. Connections required for each of the three

analog inputs, R1IN, R2IN and R3IN, and the resulting full-scale

ranges, are shown in Table I and Table II. The nominal input

impedance for each analog input range is also shown. Table III

shows the output codes for the ideal input voltages of each of the

six analog input ranges.

The analog input section has a

±25 V overvoltage protection on

R1IN and R2IN. Since the AD977/AD977A has two analog

grounds it is important to ensure that the analog input is refer-

enced 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 input of the

AD977/AD977A is 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 best match the low distortion requirements of the AD977/

AD977A, care should be taken in the selection of the drive cir-

cuitry op amp.

Figure 10 shows the simplified analog input section for the

AD977/AD977A. Since the AD977/AD977A can operate with

an internal or external reference, and several 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 onboard resistors are laser trimmed to ratio

match for adjustment of offset and full-scale error using fixed

external resistors.

The configurations shown in Figures 12 and 13 are required to

obtain the data sheet specifications for offset and full-scale error.

The external fixed resistors are used during factory calibration so

that a single 5 V supply can be used to bias the hardware trim

circuitry. With the hardware adjust circuits shown in Figures 12

and 13, offset and full-scale error can be trimmed to zero. Refer

to the Offset and Gain Adjust section.

If larger offset and full-scale errors are permitted, or if soft-

ware calibration is used, the external resistors can be omit-

ted. Table IV shows the resultant input ranges and offset and

full-scale errors.

Using the AD977A with Bipolar Input Ranges

The connection diagrams in Figure 11 show a buffer amplifier

required for bipolar operation of the AD977A when using the

internal reference. The buffer amplifier is required to isolate the

CAP pin from the signal dependent current in the R3IN pin. A

high speed op amp such as the AD8031 can be used with a

single 5 V power supply without degrading the performance of

the AD977A. The buffer must have good settling characteristics

and provide low total noise within the input bandwidth of the

AD977A.

R1IN

R2IN

R3IN

REF

AGND2

CAP

AGND1

AD977/AD977A

20k /10k

2.5V

REFERENCE

10k /5k

5k /2.5k

20k /10k

40pF

SWITCHED

CAP ADC

Figure 10. AD977/AD977A Simplified Analog Input

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