REV. A
AD974
–13–
Table I. Analog Input Configuration
Input Voltage Connect Connect Input
Range VxA to VxB to Impedance
±10 V BIP V
IN
13.7 kΩ
0 V to +5 V V
IN
GND 6.0 kΩ
0 V to +4 V V
IN
V
IN
6.4 kΩ
Table II. Output Codes and Ideal Input Voltage
Digital Input
Description Analog Input Straight Binary
Full-Scale Range ±10 V 0 V to +5 V 0 V to +4 V
Least Significant Bit 305 µV 76 µV 61 µV
+Full Scale (FS – 1 LSB) +9.999695 V +4.999847 V +3.999939 V 1111 1111 1111 1111
Midscale 0 V +2.5 V +2 V 1000 0000 0000 0000
One LSB Below Midscale –305 µV +2.499924 V +1.999939 V 0111 1111 1111 1111
–Full Scale –10 V 0 V 0 V 0000 0000 0000 0000
ANALOG INPUTS
The AD974 is specified to operate with three full-scale analog
input ranges. Connections required for each of the eight analog
inputs, VxA and VxB and the resulting full-scale ranges, are
shown in Table I. The nominal input impedance for each ana-
log input range is also shown. Table II shows the output codes
for the ideal input voltages of each of the analog input ranges.
The analog input section has a ±25␣ V overvoltage protection on
VxA and VxB. Since the AD974 has two analog grounds it is
important to ensure that the analog input is referenced to the
AGND1 pin, the low current ground. This will minimize any
problems associated with a resistive ground drop. It is also
important to ensure that the analog inputs are driven by a low
impedance source. With its primarily resistive analog input
circuitry, the ADC can be driven by a wide selection of general
purpose amplifiers.
To achieve the low distortion capability of the AD974 care
should be taken in the selection of the drive circuitry
op amp.
Figure 10 shows the simplified analog input section for the
AD974. Since the AD974 can operate with an internal or exter-
nal reference, and three different analog input ranges, the full-
scale analog input range is best represented with a voltage that
spans 0␣ V to V
REF
across the 40 pF sampling capacitor. The on-
chip resistors are laser trimmed to ratio match for adjustment of
offset and full-scale error using fixed external resistors.
BIP AGND1 REF
CAP
VxA
VxB
AGND2
3kV
12kV
4kV
SWITCHED
CAP ADC
2.5V
REFERENCE
4kV
40pF
AD974
Figure 10. Simplified Analog Input