Datasheet

Data Sheet AD7176-2
Rev. A | Page 25 of 68
AD8656
The AD8656 is a low noise, dual precision CMOS amplifier. The
AD8656 allows the user to connect a signal of interest directly
to a high impedance, low noise, low offset amplifier input that
can drive the AD7176-2 switched capacitor input. The AD8656
can operate from a single 5 V supply. When using an external
5 V reference such as the ADR445 in conjunction with the
AD7176-2, the AD8656 output can swing to within −1 dBFS
(which equates to a differential input of ±4.45 V) of the ADC
input range.
A simple configuration for use of the AD8656 is to connect the
amplifiers in a configuration for a gain of more than 1. Each of
the AD7176-2 analog inputs has its own amplifier. This allows
the user to connect either fully differential inputs or single-ended
inputs to the AD7176-2. The example shown in Figure 31 is
configured with two fully differential inputs connecting to the
AIN0/AIN1 pair and the AIN2/AIN3 pair.
The high impedance input to the amplifier allows the user to
band-limit the input with a suitable passive filter RC combination.
The gain of the configuration used is set by the R
G
and R
F
resistors.
To improve accuracy, use precision resistors for R
G
and R
F
.
Setting R
G
= R
F
= 1 kΩ results in a gain of 2 for the circuit. The
matching of the RG and RF resistors directly affects the gain
error of the circuit. The drift and matching of these resistors
affect the gain error drift of the circuit. A 10 Ω source resistor (R
S
)
is placed between the feedback resistor (R
F
) and the amplifier
output. This resistor acts to isolate the amplifier from any
kickback from the ADC input and does not directly affect the
gain error of the circuit.
The output of each of the amplifier pairs is connected directly
to a network of decoupling and differential capacitors prior to
being connected to the AD7176-2 analog inputs. The capacitor
network shown in Figure 31 includes C1, C2 = 270 pF and C3 =
680 pF. T he capacitor network acts to provide the dynamic charge
required by the AD7176-2 switched sampling capacitors.
The circuit example in Figure 31 requires inclusion of two
precision gain resistors per amplifier (R
G
and R
F
). Choose the
value, precision, and matching of such resistors according to the
requirements of the application.
Figure 31. Dual AD8656 Amplifiers Driving the AD7176-2
AD7176-2
C1
C2
C3
ADR445
2
MICROCONTROLLER
HOST
AD8656
0.1µ
F
R
F
R
G
R
S
R
S
+5V
+5V
R
F
R
G
IN0
0 TO +2.5V
INPUT RANGE
0 TO +2.5V
INPUT RANGE
IN1
5V VREF
0.1µ
F
4.7µ
F
5.5V TO 18V
0.1µF
0.1µF
2
6
3
REF–
AVSS
7
AVDD1
8
AVDD2
1
1
AVDD2 CAN BE SUPPLIED BY VOLTAGES RANGING FROM 2V TO 5.5V.
2
USING ADR444 (4.096V REFERENCE) IN PLACE OF THE ADR445 AS SHOWN IN THIS
EXAMPLE WOULD ALLOW THE ENTIRE CCT TO BE OPERATED FROM A SINGLE 5V SUPPLY RAIL.
REF+
22
21
C1
C2
C3
AD8656
R
F
R
G
R
S
R
S
0.1µF
R
F
R
G
IN2
0 TO +2.5V
INPUT RANGE
0 TO +2.5V
INPUT RANGE
IN3
AIN0
AIN1
0.1µF
0.1µF
+5V
CS
11
12
13
14
DOUT/RDY
DIN
SCLK
24
23
AIN2
AIN3
11037-158