–9–

REV. D

AD7701

The output settling of the AD7701 in response to a step input

change is shown in Figure 12. The Gaussian response has fast

settling with no overshoot, and the worst-case settling time to

±0.0007% (±0.5 LSB) is 125 ms with a 4.096 MHz master

clock frequency.

100

80

60

40

20

0

0

40

80

120

160

TIME – ms

Figure 12. AD7701 Step Response

USING THE AD7701

SYSTEM DESIGN CONSIDERATIONS

The AD7701 operates differently from successive approxima-

tion ADCs or other integrating ADCs. Since it samples the sig-

nal continuously, like a tracking ADC, there is no need for a

start convert command. The 16-bit output register is updated at

a 4 kHz rate, and the output can be read at any time, either syn-

chronously or asynchronously.

CLOCKING

The AD7701 requires a master clock input, which may be an

external TTL/CMOS compatible clock signal applied to the

CLKIN pin (CLKOUT not used). Alternatively, a crystal of the

correct frequency can be connected between CLKIN and

CLKOUT, when the clock circuit will function as a crystal-

controlled oscillator.

The input sampling frequency, output data rate, filter character-

istics and calibration time are all directly related to the master

table. Therefore, the first step in system design with the

AD7701 is to select a master clock frequency suitable for the

bandwidth and output data rate required by the application.

ANALOG INPUT RANGES

The AD7701 performs conversion relative to an externally

supplied reference voltage, which allows easy interfacing to

ratiometric systems. In addition, either unipolar or bipolar input

voltage range may be selected using the BP/UP input. With BP/

UP

tied low, the input range is unipolar and the span is 0 to

span is

±V

tracking of positive and negative full scale and better midscale

(bipolar zero) stability than bipolar schemes that simply scale

and offset the input range.

The digital output coding for the unipolar range is Unipolar

Binary; for the bipolar range it is Offset Binary. Bit weights for

the unipolar and bipolar modes are shown in Table I. The input

voltages and output codes for unipolar and bipolar ranges, using

the recommended +2.5 V reference, are shown in Table II.

Table I. Bit Weight Table (2.5 V Reference Voltage)

Unipolar Mode

Bipolar Mode

V

LSBs

% FS

ppm FS

LSBs % FS

ppm FS

10

0.26

0.0004

4

0.13

0.0002

2

19

0.5

0.0008

8

0.26

0.0004

4

38

1.00

0.0015

15

0.5

0.0008

8

76

2.00

0.0031

31

1.00

0.0015

15

153

4.00

0.0061

61

2.00

0.0031

31

Table II. Output Coding

Unipolar Mode

Bipolar Mode

Input Relative to

Input in Volts

Input Relative to

Input in Volts

Output Data

FS and AGND

FS and AGND

1111 1111 1111 1111

+2.499943

+2.499886

1111 1111 1111 1110

+2.499905

+2.499810

1111 1111 1111 1101

+2.499867

+2.499733

1111 1111 1111 1100

1000 0000 0000 0001

+1.250019

AGND + 0.5 LSB

+0.000038

1000 0000 0000 0000

+1.249981

AGND – 0.5 LSB

–0.000038

0111 1111 1111 1111

+1.249943

AGND – 1.5 LSB

–0.000114

0111 1111 1111 1110

0000 0000 0000 0011

AGND + 2.5 LSB

+0.000095

–2.499810

0000 0000 0000 0010

AGND + 1.5 LSB

+0.000057

–2.499886

0000 0000 0000 0001

AGND + 0.5 LSB

+0.000019

–2.499962

0000 0000 0000 0000

NOTES