AD7691 Analog Devices, AD7691 Datasheet - Page 16

• Manufacturer Part Number: AD7691
• Manufacturer: Analog Devices

Specifications of AD7691

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 250kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: CSP,SOP

AD7691
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
For ac applications, the driver should have a THD performance
For multichannel multiplexed applications, the driver amplifier
Table 10. Recommended Driver Amplifiers
Amplifier
ADA4941-1
ADA4841-x
AD8655
AD8021
AD8022
OP184
AD8605,
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 R
external filter, if one is used. The SNR degradation due to
the amplifier is as follows:
SNR
where:
V
f
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).
e
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.
commensurate with the AD7691.
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.
20
V
−3 dB
N+
NADC
NADC
log
and e
AD8615
LOSS
is the input bandwidth, in MHz, of the AD7691 (2 MHz)
is the noise of the ADC, in μV, given by the following:
⎛
⎜
⎜
⎜
⎜
⎝
=
=
N−
10
V
V
2
are the equivalent input noise voltage densities
INpp
NADC
SNR
20
2
Typical Application
Very low noise, low power single-ended-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
2
+
π
2
f
−
3
dB
(
V
Ne
NADC
N
+
IN
)
and C
2
+
π
2
IN
f
−
3
or by the
dB
(
Ne
N
−
)
2
⎞
⎟
⎟
⎟
⎟
⎠
Rev. B | Page 16 of 28
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.
R1 and R2 set the attenuation ratio between the input range and
the ADC range (V
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 V
supply is desired, it can be set slightly above V
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
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
If desired, smaller reference decoupling capacitor values as low
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.
±10V, ±5V, ...
100nF
100nF
R5
R3
Figure 32. Single-Ended-to-Differential Driver Circuit
R1
REF
R6
R4
). R1, R2, and C
+5.2V
R2
C
F
ADA4941
15Ω
15Ω
2.7nF
2.7nF
F
AD8031
are chosen depending on
REF
/2; however, if single
10µF
ADR43x
IN+
IN–
REF
or the AD8605, a
GND
/2 to provide
REF
AD7691
reference.
+5V REF
+5.2V
VDD