AD7634 Analog Devices, AD7634 Datasheet - Page 21

• Manufacturer Part Number: AD7634
• Manufacturer: Analog Devices

Specifications of AD7634

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 670kSPS
• Interface: Byte,Par,Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: 10V p-p,20 V p-p,40 V p-p
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

DRIVER AMPLIFIER CHOICE
Although the AD7634 is easy to drive, the driver amplifier must
meet the following requirements:
•
•
• The driver needs to have a THD performance suitable to
The AD8021 meets these requirements and is appropriate for
almost all applications. The AD8021 needs a 10 pF external
compensation capacitor that should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
+1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
The
and a gain of 1 is present. The
cations where high frequency (above 100 kHz) performance is not
required. In applications with a gain of 1, an 82 pF compensation
N is the noise factor of the amplifier (+1 in buffer
configuration).
e
of the op amps connected to IN+ and IN−, in nV/√Hz.
This approximation can be utilized when the resistances
used around the amplifiers are small. If larger resistances are
used, their noise contributions should also be root-sum
squared.
that of the AD7634. Figure 15 shows the THD vs. frequency
that the driver should exceed.
AD8022
For multichannel, multiplexed applications, the driver ampli-
fier and the AD7634 analog input circuit must be able to
settle for a full-scale step of the capacitor array at a 18-bit
level (0.0004%). For the amplifier, settling at 0.1% to 0.01%
is more commonly specified. This differs significantly from
the settling time at a 18-bit level and should be verified
prior to driver selection. The
low noise and high gain bandwidth and meets this settling
time requirement even when used with gains of up to 13.
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 AD7634. The noise coming from
the driver is filtered by the external 1-pole low-pass filter,
as shown in Figure 27. The SNR degradation due to the
amplifier is
where:
N+
V
V
f
SNR
–3dB
NADC
NADC
and e
is the cutoff frequency of the input filter (3.9 MHz).
LOSS
is the noise of the ADC, which is:
=
N−
can also be used when a dual version is needed
=
2
10
V
are the equivalent input voltage noise densities
2
20
INp-p
SNR
20
log
2
⎛
⎜
⎜
⎜
⎜ ⎜
⎝
V
NADC
AD829
2
+
AD8021
π
2
f
−
3
is an alternative in appli-
dB
(
op amp combines ultra-
V
Ne
NADC
N
+
)
2
+
π
2
f
−
3
dB
(
Ne
Rev. A | Page 21 of 32
N
−
)
2
⎞
⎟
⎟
⎟
⎟ ⎟
⎠
capacitor is required. The
current is needed in low frequency applications.
Because the AD7634 uses a large geometry, high voltage input
switch, the best linearity performance is obtained when using
the amplifier at its maximum full power bandwidth. Gaining
the amplifier to make use of the more dynamic range of the
ADC results in increased linearity errors. For applications
requiring more resolution, the use of an additional amplifier
with gain should precede a unity follower driving the AD7634.
See Table 9 for a list of recommended op amps.
Table 9. Recommended Driver Amplifiers
Amplifier
AD829
AD8021
AD8022
ADA4922-1
AD8610/
AD8620
Single-to-Differential Driver
For single-ended sources, a single-to-differential driver, such
as the ADA4922-1, can be used because the AD7634 needs to
be driven differentially. The 1-pole filter using R = 15 Ω and
C = 2.7 nF provides a corner frequency of 3.9 MHz.
For unipolar 5 V and 10 V input ranges, the internal (or
external) reference source can be used to level shift U2 fo
the correct input span. If using an external reference, the
values for R1/R2 can be lowered to reduce resistive Johnso
noise (1.29E − 10 × √R). For the bipolar ±5 V and ±10 V inpu
ranges, the reference connection is not required because the
common-mode voltage is 0 V. See Table 10 for R1/R2 for the
different input ranges.
Table 10. R1/R2 Configuration
Input Range
5 V
10 V
±5 V, ±10 V
ANALOG
INPUT
Figure 31. Single-to-Differenti
IN
ADA4922-1
Typical Application
±15 V supplies, very low noise, low frequency
±12 V supplies, very low noise, high frequency
±12 V supplies, very low noise, high
frequency, dual
±12 V supplies, low noise, high frequency,
single-ended-to-differential driver
±13 V supplies, low bias current, low
frequency, single/dual
R1
2.5 kΩ
2.5 kΩ
REF
R
R2
G
AD8610
U2
R
F
100nF
R2
2.5 k
Open
100 Ω
R1
al D
Ω
OUT+
OUT–
is an option when low bias
river Using the ADA4922-1
Common-Mode Voltage
2.5 V
5 V
0 V
2.7nF
2.7nF
15Ω
15Ω
VCC
VEE
AD7634
IN+
IN–
AD7634
10µF
r
REF
n
t