AD977/AD977A
–14–
REV. D
Table I. AD977A Analog Input Configuration
Input Voltage Connect R1
IN
Connect R2
IN
Connect R3
IN
Input
Range via 200 to via 100 to to Impedance
± 10 V V
IN
AGND 2.5 V 11.5 kΩ
± 5 V AGND V
IN
2.5 V 6.7 kΩ
± 3.3 V V
IN
V
IN
2.5 V 5.4 kΩ
0 V to 10 V AGND V
IN
AGND 6.7 kΩ
0 V to 5 V AGND AGND V
IN
5.0 kΩ
0 V to 4 V V
IN
AGND V
IN
5.4 kΩ
Table II. AD977 Analog Input Configuration
Input Voltage Connect R1
IN
Connect R2
IN
Connect R3
IN
Input
Range via 200 to via 100 to to Impedance
± 10 V V
IN
AGND CAP 22.9 kΩ
± 5 V AGND V
IN
CAP 13.3 kΩ
± 3.3 V V
IN
V
IN
CAP 10.7 kΩ
0 V to 10 V AGND V
IN
AGND 13.3 kΩ
0 V to 5 V AGND AGND V
IN
10.0 kΩ
0 V to 4 V V
IN
AGND V
IN
10.7 kΩ
ANALOG INPUTS
The AD977/AD977A is specified to operate with six full-scale
analog input ranges. Connections required for each of the three
analog inputs, R1
IN
, R2
IN
and R3
IN
, and the resulting full-scale
ranges, are shown in Table I and Table II. The nominal input
impedance for each analog input range is also shown. Table III
shows the output codes for the ideal input voltages of each of the
six analog input ranges.
The analog input section has a ± 25 V overvoltage protection on
R1
IN
and R2
IN
. Since the AD977/AD977A has two analog
grounds it is important to ensure that the analog input is refer-
enced 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 input of the
AD977/AD977A is 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 best match the low distortion requirements of the AD977/
AD977A, care should be taken in the selection of the drive cir-
cuitry op amp.
Figure 10 shows the simplified analog input section for the
AD977/AD977A. Since the AD977/AD977A can operate with
an internal or external reference, and several 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 onboard resistors are laser trimmed to ratio
match for adjustment of offset and full-scale error using fixed
external resistors.
The configurations shown in Figures 12 and 13 are required to
obtain the data sheet specifications for offset and full-scale error.
The external fixed resistors are used during factory calibration so
that a single 5 V supply can be used to bias the hardware trim
circuitry. With the hardware adjust circuits shown in Figures 12
and 13, offset and full-scale error can be trimmed to zero. Refer
to the Offset and Gain Adjust section.
If larger offset and full-scale errors are permitted, or if soft-
ware calibration is used, the external resistors can be omit-
ted. Table IV shows the resultant input ranges and offset and
full-scale errors.
Using the AD977A with Bipolar Input Ranges
The connection diagrams in Figure 11 show a buffer amplifier
required for bipolar operation of the AD977A when using the
internal reference. The buffer amplifier is required to isolate the
CAP pin from the signal dependent current in the R3
IN
pin. A
high speed op amp such as the AD8031 can be used with a
single 5 V power supply without degrading the performance of
the AD977A. The buffer must have good settling characteristics
and provide low total noise within the input bandwidth of the
AD977A.
R1
IN
R2
IN
R3
IN
REF
4k
AGND2
CAP
AGND1
AD977/AD977A
20k /10k
2.5V
REFERENCE
10k /5k
5k /2.5k
20k /10k
40pF
SWITCHED
CAP ADC
Figure 10. AD977/AD977A Simplified Analog Input
