AD711
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
Figure 31. Unipolar Binary Operation
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 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
of the AD7545 for V
DD
= +5 V
TRIM
RESISTOR JN/AQ/SD KN/BQ/TD LN/CQ/UD GLN/GCQ/GUD
R1
R2
500
Ω
150
Ω
200
Ω
68
Ω
100
Ω
33
Ω
20
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.
Figure 34. AD711 as ADC Unity Gain Buffer
a. Full-Scale Positive
Transition
a. Full-Scale Positive
Transition
b. Full-Scale Negative
Transition
b. Full-Scale Negative
Transition
a. Source Current = 2 mA
b. Sink Current = 1 mA
Figure 33. Settling Characteristics for AD711 with AD7545
Figure 35. ADC Input Unity Gain Buffer Recovery Times
–10–
REV. A
AD [ ANALOG DEVICES ]
