AD977AR Analog Devices Inc, AD977AR Datasheet - Page 14

• Manufacturer Part Number: AD977AR
• Description: A/D Converter (A-D) IC
• Manufacturer: Analog Devices Inc

Specifications of AD977AR

• Package/case: 20-SOIC
• Features: 16?Bit, 100kSPS, BiCMOS ADC
• Interface Type: Serial
• Leaded Process Compatible: No
• Number Of Bits: 16
• Number Of Channels: 1
• Peak Reflow Compatible (260 C): No
• Rohs Status: RoHS non-compliant
• Sampling Rate (per Second): 100k
• Data Interface: Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 100mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (0.300", 7.50mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With: EVAL-AD977CB - BOARD EVAL FOR AD977EVAL-AD977ACB - BOARD EVAL FOR AD977A
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD977/AD977A
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
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
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
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
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
IN
and R2
IN
. Since the AD977/AD977A has two analog
IN
, R2
IN
and R3
Connect R1
via 200
V
AGND
V
AGND
AGND
V
Connect R1
via 200
V
AGND
V
AGND
AGND
V
IN
IN
IN
IN
IN
IN
IN
REF
, and the resulting full-scale
across the 40 pF sampling
to
to
IN
IN
Table I. AD977A Analog Input Configuration
Table II. AD977 Analog Input Configuration
Connect R2
via 100
AGND
V
V
V
AGND
AGND
Connect R2
via 100
AGND
V
V
V
AGND
AGND
IN
IN
IN
IN
IN
IN
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
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.
to
to
IN
IN
AGND2
R1
R2
R3
CAP
IN
IN
IN
20k /10k
10k /5k
5k /2.5k
AD977/AD977A
AGND1
Connect R3
to
2.5 V
2.5 V
2.5 V
AGND
V
V
Connect R3
to
CAP
CAP
CAP
AGND
V
V
IN
IN
IN
IN
20k /10k
REF
IN
IN
4k
SWITCHED
CAP ADC
40pF
REFERENCE
2.5V
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
IN
pin. A