AD570

REV. A

–4–

FULL-SCALE CALIBRATION

The 5 k

Ω thin-film input resistor is laser trimmed to produce a

current which matches the full-scale current of the internal

DAC—plus about 0.3%—when a full-scale analog input voltage

of 9.961 volts (10 volts—1 LSB) is applied at the input. The in-

put resistor is trimmed in this way so that if a fine trimming

potentiometer is inserted in series with the input signal, the in-

put current at the full-scale input voltage can be trimmed down

to match the DAC full-scale current as precisely as desired.

However, for many applications the nominal 9.961 volt full

scale can be achieved to sufficient accuracy by simply inserting a

15

Ω resistor in series with the analog input to Pin 13. Typical

full-scale calibration error will then be about

±2 LSB or ±0.8%.

If a more precise calibration is desired, a 200

Ω trimmer should

be used instead. Set the analog input at 9.961 volts, and set the

trimmer so that the output code is just at the transition between

11111110 and 11111111. Each LSB will then have a weight of

39.06 mV. If a nominal full scale of 10.24 volts is desired

(which makes the LSB have a value of exactly 40.00 mV), a

50

Ω resistor in series with a 200 Ω trimmer (or a 500 Ω trim-

mer with good resolution) should be used. Of course, larger full-

scale ranges can be arranged by using a larger input resistor, but

linearity and full-scale temperature coefficient may be compro-

mised if the external resistor becomes a sizable percentage of

5 k

Ω.

BIPOLAR OPERATION

The standard unipolar 0 V to +10 V range is obtained by short-

ing the bipolar offset control pin to digital common. If the pin is

left open, the bipolar offset current will be switched into the

comparator summing node, giving a –5 V to +5 V range with an

Figure 4. Standard AD570 Connections

offset binary output code. (–5.00 volts in will give a 8-bit code

of 00000000; an input of 0.00 volts results in an output code of

10000000 and 4.96 volts at the input yields the 11111111 code.)

The bipolar offset control input is not directly TTL compatible,

but a TTL interface for logic control can be constructed as

shown in Figure 5.

Figure 5. Bipolar Offset Controlled by Logic Gate

Gate Output = 1: Unipolar 0 V–10 V Input Range

Gate Output = 0: Bipolar

±5 V Input Range

COMMON-MODE RANGE

The AD570 provides separate analog and digital common con-

nections. The circuit will operate properly with as much as

±200 mV of common-mode range between the two commons.

This permits more flexible control of system common bussing

and digital and analog returns.

In normal operation the analog common terminal may generate

transient currents of up to 2 mA during a conversion. In addi-

tion, a static current of about 2 mA will flow into analog com-

mon in the unipolar mode after a conversion is complete. An

additional 1 mA will flow in during a blank interval with zero

analog input. The analog common current will be modulated by

the variations in input signal.

The absolute maximum differential voltage rating between the

two commons is

±1 volt. We recommend that a parallel pair of

back-to-back protection diodes can be connected as shown in

Figure 6 if they are not connected locally.

Figure 6. Differential Common Voltage Protection