AD711

REV. A

–10–

Figure 31. Unipolar Binary Operation

Figure 32. Bipolar Operation

R1 and R2 calibrate the zero offset and gain error of the DAC.

Specific values for these resistors depend upon the grade of

AD7545 and are shown below.

Table I. Recommended Trim Resistor Values vs. Grades

TRIM

RESISTOR JN/AQ/SD KN/BQ/TD LN/CQ/UD GLN/GCQ/GUD

R1

500

Ω

200

Ω

100

Ω

20

Ω

R2

150

Ω

68

Ω

33

Ω

6.8

Ω

Figures 33a and 33b show the settling time characteristics of the

AD711 when used as a DAC output buffer for the AD7545.

a. Full-Scale Positive

b. Full-Scale Negative

Transition

Transition

Figure 33. Settling Characteristics for AD711 with AD7545

NOISE CHARACTERISTICS

The random nature of noise, particularly in the 1/f region, makes

it difficult to specify in practical terms. At the same time, design-

ers of precision instrumentation require certain guaranteed maxi-

mum noise levels to realize the full accuracy of their equipment.

The AD711C grade is specified at a maximum level of 4.0

µV

p-p, in a 0.1 to 10 Hz bandwidth. Each AD711C receives a

100% noise test for two 10-second intervals; devices with any ex-

cursion in excess of 4.0

µV are rejected. The screened lot is then

submitted to Quality Control for verification on an AQL basis.

All other grades of the AD711 are sample-tested on an AQL

basis to a limit of 6

µV p-p, 0.1 to 10 Hz.

DRIVING THE ANALOG INPUT OF AN A/D CONVERTER

An op amp driving the analog input of an A/D converter, such as

that shown in Figure 34, must be capable of maintaining a con-

stant output voltage under dynamically changing load condi-

tions. In successive-approximation converters, the input current

is compared to a series of switched trial currents. The compari-

son point is diode clamped but may deviate several hundred mil-

livolts resulting in high frequency modulation of A/D input

current. The output impedance of a feedback amplifier is made

artificially low by the loop gain. At high frequencies, where the

loop gain is low, the amplifier output impedance can approach

its open loop value. Most IC amplifiers exhibit a minimum open

loop output impedance of 25

Ω due to current limiting resistors.

A few hundred microamps reflected from the change in converter

Figure 34. AD711 as ADC Unity Gain Buffer

a. Full-Scale Positive

Transition

b. Full-Scale Negative

Transition

Figure 35. ADC Input Unity Gain Buffer Recovery Times

a. Source Current = 2 mA

b. Sink Current = 1 mA