AD977ABRZ Datasheet, PDF (14/24 Pages) Analog Devices – 16-Bit, 100 kSPS/200 kSPS BiCMOS A/D Converter

AD977/AD977A
Input Voltage
Range
± 10 V
±5 V
± 3.3 V
0 V to 10 V
0 V to 5 V
0 V to 4 V
Input Voltage
Range
± 10 V
±5 V
± 3.3 V
0 V to 10 V
0 V to 5 V
0 V to 4 V
Table I. AD977A Analog Input Configuration
Connect R1_IN
via 200 ⍀ to
V_IN
AGND
V_IN
AGND
AGND
V_IN
Connect R2_IN
via 100 ⍀ to
AGND
V_IN
V_IN
V_IN
AGND
AGND
Connect R3_IN
to
2.5 V
2.5 V
2.5 V
AGND
V_IN
V_IN
Table II. AD977 Analog Input Configuration
Connect R1_IN
via 200 ⍀ to
V_IN
AGND
V_IN
AGND
AGND
V_IN
Connect R2_IN
via 100 ⍀ to
AGND
V_IN
V_IN
V_IN
AGND
AGND
Connect R3_IN
to
CAP
CAP
CAP
AGND
V_IN
V_IN
Input
Impedance
11.5 kΩ
6.7 kΩ
5.4 kΩ
6.7 kΩ
5.0 kΩ
5.4 kΩ
Input
Impedance
22.9 kΩ
13.3 kΩ
10.7 kΩ
13.3 kΩ
10.0 kΩ
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.
AGND1
REF
CAP
R1_IN
R2_IN
R3_IN
20k /10k
10k /5k
5k /2.5k
4k
2.5V
REFERENCE
SWITCHED
CAP ADC
20k /10k
40pF
AGND2 _AD977/AD977A
Figure 10. AD977/AD977A Simplified Analog Input
–14–
REV. D