Datasheet

AD7766
Rev. C | Page 20 of 24
DRIVING THE AD7766/AD7766-1/AD7766-2
R1 and R2 set the attenuation ratio between the input range and
the ADC range (V
REF+
). R1, R2, and C
F
are chosen depending on
the desired input resistance, signal bandwidth, antialiasing, and
noise contribution. The ratio of R2 to R1 should be equal to the
ratio of REF to the peak-to-peak input voltage. For example, for
the ±10 V range with a 4 kΩ impedance, R2 = 1 kΩ and R1 = 4 kΩ.
The AD7766/AD7766-1/AD7766-2 must be driven with fully
differential inputs. The common-mode voltage of the differential
inputs to the AD7766/AD7766-1/AD7766-2 devices, and therefore
the limits on the differential inputs, is set by the reference voltage
(V
REF+
) applied to the device. The common-mode voltage of the
AD7766/AD7766-1/AD7766-2 is V
REF+
/2. When the AD7766/
AD7766-1/AD7766-2 V
REF+
pin has a 5 V supply (using ADR445,
ADR435, or ADR425), the common mode is at 2.5 V, meaning
that the maximum inputs that can be applied on the AD7766/
AD7766-1/AD7766-2 differential inputs are a 5 V p-p input
around 2.5 V.
R3 and R4 set the common mode on the V
IN−
input, and R5 and R6
set the common mode on the V
IN+
input of the ADC. The common
mode, which is equal to the voltage present at V
OFFSET1
, should be
close to V
REF+
/2. The voltage present should roughly be set to the
ratio of V
OFFSET1
to 1 + R2/R1.
06449-016
0V
V
IN+
V
IN–
V
REF
V
REF
2
0V
V
REF
V
REF
2
06449-017
ADA4841-1
15Ω
3.3nF
1kΩ
ADA4841-1
15Ω
3.3nF
1kΩ
4
5
1
2
AD7766
AV
DD
V
REF+
V
IN+
V
IN–
2.2nF
2.2nF
*
ADR4xx
ADP3330-2.5
2.5V
2.5V TO 5V
REFERENCE
VOLTAGE
1kΩ
1kΩ
1kΩ
1kΩ
A
IN+
AIN–
*
SEE V
REF+
INPUT SIGNAL SECTION FOR DETAILS.
Figure 39. Maximum Differential Inputs to the AD7766
An analog voltage of 2.5 V supplies the AD7766/AD7766-1/
AD7766-2 AV
DD
pin. However, the AD7766/AD7766-1/AD7766-2
allow the user to apply a reference voltage of up to 5 V. This
provides the user with an increased full-scale range, offering the
user the option of using the AD7766/AD7766-1/AD7766-2 with
a greater LSB voltage. Figure 39 shows the maximum inputs to
the AD7766.
Figure 40. Driving the AD7766 from a Fully Differential Source
15Ω
15Ω
100µF
R1
100nF
100nF
2.5V
VOUT = 5V REF
5.2V
–0.2V
C
F
R4
R2
R6
V
IN
R3
R5
V
REF+ AV
DD
AGND
V
IN+
V
IN–
AD7766
2.2nF
2.2nF
ADA4941-1
IN
FB
OUTP
OUTN
REF
V
OFFSET1
V
OFFSET2
ADR425
ADR445
V
IN
ADP3330-2.5
LDO
DGND
0.1µF
0.1µF
06449-018
DIFFERENTIAL SIGNAL SOURCE
An example of recommended driving circuitry that can be used
in conjunction with the AD7766 is shown in Figure 40. Figure 40
shows how the ADA4841-1 device can be used to drive an input
to the AD7766 from a differential source. Each of the differential
paths is driven by an ADA4841-1 device.
SINGLE-ENDED SIGNAL SOURCE
For applications using a single-ended analog signal, either
bipolar or unipolar, the ADA4941-1 single-ended-to-differential
driver creates a fully differential input to the AD7766. The
schematic is shown in Figure 41.
Figure 41. Driving the AD7766 from a Single-Ended Source
Table 8. Resistor Values Required When Using the Differential-to-Single-Ended Circuit with ADA4941 (See Figure 41)
V
IN
(V) V
OFFSET1
(V) V
OFFSET2
(V) OUT+ (V) OUT− (V) R1 (kΩ) R2 (kΩ) R4 (kΩ) R3 = R5 = R6 (kΩ)
+20, −20 2.5 2.203 −0.01, +4.96 5.01, 0.04 8.06 1 12.7 10
+10, −10 2.5 2.000 0.01, 4.99 4.99, 0.01 4.02 1 15 10
+5, −5 2.5 1.667 0.00, 5.00 5.00, 0.00 2 1 20 10