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AD7690
Rev. B | Page 15 of 24
For ac applications, the driver should have a THD
performance commensurate with the AD7690.
For multichannel multiplexed applications, the driver
amplifier and the AD7690 analog input circuit must settle
for a full-scale step onto the capacitor array at an 18-bit
level (0.0004%, 4 ppm). In the amplifier’s data sheet, settling
at 0.1% to 0.01% is more commonly specified. This may
differ significantly from the settling time at an 18-bit level
and should be verified prior to driver selection.
Table 8. Recommended Driver Amplifiers
Amplifier
Typical Application
Very low noise, low power single to differential
Very low noise, small, and low power
5 V single supply, low noise
Very low noise and high frequency
Low noise and high frequency
Low power, low noise, and low frequency
5 V single supply, low power
SINGLE-TO-DIFFERENTIAL DRIVER
For applications using a single-ended analog signal, either bipolar
or unipolar, the
ADA4941-1 single-ended-to-differential driver
allows for a differential input into the part. The schematic is
R1 and R2 set the attenuation ratio between the input range and
the ADC range (VREF). R1, R2, and CF are chosen depending on
the desired input resistance, signal bandwidth, antialiasing, and
noise contribution. For example, for the ±10 V range with a 4 kΩ
impedance, R2 = 1 kΩ and R1 = 4 kΩ.
R3 and R4 set the common mode on the IN input, and R5 and
R6 set the common mode on the IN+ input of the ADC. The
common mode should be set close to VREF/2; however, if single
supply is desired, it can be set slightly above VREF/2 to provide
some headroom for the
ADA4941-1 output stage. For example,
for the ±10 V range with a single supply, R3 = 8.45 kΩ, R4 =
11.8 kΩ, R5 = 10.5 kΩ, and R6 = 9.76 kΩ.
AD7690
REF
GND
VDD
IN+
2.7nF
100nF
2.7nF
IN–
+5V REF
±10V, ±5V, ...
+5.2V
15
10F
15
R2
CF
ADA4941
R1
R3
100nF
R5
R4
R6
05
79
2-
01
0
Figure 30. Single-Ended-to-Differential Driver Circuit
VOLTAGE REFERENCE INPUT
The AD7690 voltage reference input, REF, has a dynamic input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins, as explained in the
Layout section.
When REF is driven by a very low impedance source (for
a 10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate
for optimum performance.
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift
ADR43x reference.
If desired, a reference-decoupling capacitor with a value as small
as 2.2 μF can be used with a minimal impact on performance,
especially DNL.
Regardless, there is no need for an additional lower value
ceramic decoupling capacitor (for example, 100 nF) between the
REF and GND pins.