EVAL-SDP-CB1Z View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-SDP-CB1Z' PDF : 28 Pages View PDF
AD7691
Data Sheet
DRIVER AMPLIFIER CHOICE
Although the AD7691 is easy to drive, the driver amplifier must
meet the following requirements:
The noise generated by the driver amplifier needs to be
kept as low as possible to preserve the SNR and transition
noise performance of the AD7691. The noise coming from
the driver is filtered by the AD7691 analog input circuit’s
1-pole, low-pass filter made by RIN and CIN or by the
external filter, if one is used. The SNR degradation due to
the amplifier is as follows:
SNRLOSS =
20
log
V NADC
VNADC 2
π
2
f
3
dB
(Ne N
)2
π
2
f 3 dB (Ne N )2
where:
VNADC is the noise of the ADC, in μV, given by the following:
VINpp
VNADC
22
SNR
10 20
f−3 dB is the input bandwidth, in MHz, of the AD7691 (2 MHz)
or the cutoff frequency of the input filter, if one is used.
N is the noise gain of the amplifier (for example, 1 in
buffer configuration).
eN+ and eN− are the equivalent input noise voltage densities
of the op amps connected to IN+ and IN−, in nV/√Hz.
This approximation can be used when the resistances around
the amplifier are small. If larger resistances are used, their
noise contributions should also be root-sum-squared.
For ac applications, the driver should have a THD
performance commensurate with the AD7691.
For multichannel multiplexed applications, the driver
amplifier and the AD7691 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 10. Recommended Driver Amplifiers
Amplifier
Typical Application
ADA4941-1
Very low noise, low power single-ended-to-
differential
ADA4841-2
Very low noise, small, and low power
AD8655
5 V single supply, low noise
AD8021
Very low noise and high frequency
AD8022
Low noise and high frequency
OP184
Low power, low noise, and low frequency
AD8605, AD8615 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 shown in Figure 32.
R5
R6
R3
100nF
100nF
R4
+5.2V
10µF
+5V REF
+5.2V
15Ω
2.7nF
2.7nF
15Ω
ADA4941
REF
IN+
VDD
AD7691
IN–
GND
±10V, ±5V, ...
R1
R2
CF
Figure 32. Single-Ended-to-Differential Driver Circuit
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.
VOLTAGE REFERENCE INPUT
The AD7691 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
example, a reference buffer using the AD8031 or the AD8605, 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 ADR431, ADR433,
ADR434, and ADR435 reference.
If desired, smaller reference decoupling capacitor values as low
as 2.2 μF can be used with a minimal impact on performance,
especially DNL.
Rev. E | Page 16 of 28
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