AD571

REV. A

–5–

NOTE: During a conversion transient currents from the analog

common terminal will disturb the offset voltage. Capacitive de-

coupling should not be used around the offset network. These

transients will settle as appropriate during a conversion. Capaci-

tive decoupling will “pump up” and fail to settle resulting in

conversion errors. Power supply decoupling which returns to

analog signal common should go to the signal input side of the

resistive offset network.

0000000100

0000000011

0000000010

0000000001

0000000000

0V 10mV

30mV

50mV

INPUT VOLTAGE

OUTPUT

CODE

NORMAL CHARACTERISTICS

REFERRED TO ANALOG COMMON

0000000100

0000000011

0000000010

0000000001

0000000000

0V 10mV

30mV

50mV

INPUT VOLTAGE

OUTPUT

CODE

OFFSET CHARACTERISTICS WITH

2.7

Ω IN SERIES WITH ANALOG COMMON

Figure 8. AD571 Transfer Curve—Unipolar Operation

(Approximate Bit Weights Shown for Illustration, Nominal

Bit Weights

9.766 mV)

BIPOLAR CONNECTION

To obtain the bipolar –5 V to +5 V range with an offset binary

output code the bipolar offset control pin is left open.

A –5.0 volt signal will give a 10-bit code of 0000000000; an in-

put of 0.00 volts results in an output code of 1000000000;

+4.99 volts at the input yields 1111111111. The nominal trans-

fer curve is shown in Figure 9.

INPUT VOLTAGE – mV

0

–30

–20

–10

0

+10

+20

+30

10000 00000

01111 11111

01111 11110

10000 00010

10000 00001

OUTPUT

CODE

Figure 9. AD571 Transfer Curve—Bipolar Operation

ZERO OFFSET

The apparent zero point of the AD571 can be adjusted by

inserting an offset voltage between the analog common of the

device and the actual signal return or signal common. Figure 7

illustrates two methods of providing this offset. Figure 7a shows

how the converter zero may be offset by up to

±3 bits to correct

the device initial offset and/or input signal offsets. As shown, the

circuit gives approximately symmetrical adjustment in unipolar

mode. In bipolar mode R2 should be omitted to obtain a sym-

metrical range.

AD571

INPUT

SIGNAL

R1

10

Ω

R2

7.5k

Ω

R3

4.7k

Ω

R4

10k

Ω

SIGNAL COMMON

+15V

ZERO OFFSET ADJ

±3 BIT RANGE

–15V

Figure 7a.

AD571

INPUT

SIGNAL

SIGNAL COMMON

R1

2.7

Ω OR

5

Ω POT

1/2 BIT ZERO OFFSET

Figure 7b.

Figure 8 shows the nominal transfer curve near zero for an

AD571 in unipolar mode. The code transitions are at the edges

of the nominal bit weights. In some applications it will be pref-

erable to offset the code transitions so that they fall between the

nominal bit weights, as shown in the offset characteristics. This

offset can easily be accomplished as shown in Figure 7b. At bal-

ance (after a conversion) approximately 2 mA flows into the

analog common terminal. A 2.7

Ω resistor in series with this

terminal will result in approximately the desired 1/2 bit offset of

the transfer characteristics. The nominal 2 mA analog common

current is not closely controlled in production. If high accuracy

is required, a 5

Ω potentiometer (connected as a rheostat) can

be used as R1. Additional negative offset range may be obtained

by using larger values of R1. Of course, if the zero transition

point is changed, the full-scale transition point will also move.

Thus, if an offset of 1/2 LSB is introduced, full-scale trimming

as described on previous page should be done with an analog in-

put of 9.985 volts.