AD7622 Analog Devices, AD7622 Datasheet - Page 17

• Manufacturer Part Number: AD7622
• Manufacturer: Analog Devices

Specifications of AD7622

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 2MSPS
• Interface: Byte,Par,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

TYPICAL CONNECTION DIAGRAM
Figure 24 shows a typical connection diagram for the AD7622.
Different circuitry shown in this diagram is optional and is
discussed in the following sections.
ANALOG INPUTS
Figure 25 shows an equivalent circuit of the input structure of
the AD7622.
The two diodes, D
analog inputs IN+ and IN−. Care must be taken to ensure that
the analog input signal never exceeds the supply rails by more
than 0.3 V, because this causes the diodes to become forward-
biased and to start conducting current. These diodes can handle
a forward-biased current of 100 mA maximum. For instance,
these conditions could eventually occur when the input buffer’s
U1 or U2 supplies are different from AVDD. In such a case, an
input buffer with a short-circuit current limitation can be used
to protect the part.
The analog input of the AD7622 is a true differential structure.
By using this differential input, small signals common to both
inputs are rejected, as shown in Figure 26, representing the
typical CMRR over frequency with internal and external references.
During the acquisition phase for ac signals, the impedance of
the analog inputs, IN+ and IN−, can be modeled as a parallel
combination of capacitor C
series connection of R
capacitance. R
comprised of some serial resistors and the on resistance of the
IN+ OR IN–
75
70
65
60
55
50
45
AGND
1
Figure 26. Analog Input CMRR vs. Frequency
Figure 25. AD7622 Simplified Analog Input
IN
is typically 175 Ω and is a lumped component
1
and D
C
10
PIN
IN
and C
2
FREQUENCY (kHz)
, provide ESD protection for the
AVDD
PIN
EXT REF
D
D
and the network formed by the
IN
1
2
. C
100
PIN
INT REF
is primarily the pin
R
1000
IN
C
IN
10000
Rev. 0 | Page 17 of 28
switches. C
capacitor. During the conversion phase, when the switches are
opened, the input impedance is limited to C
make a 1-pole, low-pass filter that has a typical −3 dB cutoff
frequency of 50 MHz, thereby reducing an undesirable aliasing
effect and limiting the noise coming from the inputs.
Because the input impedance of the AD7622 is very high, the
AD7622 can be directly driven by a low impedance source
without gain error. To further improve the noise filtering achieved
by the AD7622’s analog input circuit, an external 1-pole RC
filter between the amplifier’s outputs and the ADC analog
inputs can be used, as shown in Figure 24. However, large source
impedances significantly affect the ac performance, especially
the total harmonic distortion (THD). The maximum source
impedance depends on the amount of THD that can be
tolerated. The THD degrades as a function of the source
impedance and the maximum input frequency, as shown in
Figure 27.
MULTIPLEXED INPUTS
When using the full 2 MSPS throughput in multiplexed
applications for a full-scale step, the RC filter, as shown in
Figure 24, does not settle in the required acquisition time, t
These values are chosen to optimize the best SNR performance
of the AD7622. To use the full 2 MSPS throughput in multiplexed
applications, the RC should be adjusted to satisfy t
~ 7 × RC time constant). However, lowering R and C increases
the RC filter bandwidth and allows more noise into the AD7622,
which degrades SNR. To preserve the SNR performance in these
applications using the RC filter shown in Figure 24, the AD7622
should be run with t
1.55 MSPS in wideband and warp modes.
Figure 27. THD vs. Analog Input Frequency and Source Resistance
–100
–105
–60
–65
–70
–75
–80
–85
–90
–95
1
IN
is typically 12 pF and is mainly the ADC sampling
8
> 280 ns; or approximately 1/(t
INPUT FREQUENCY (kHz)
10
R
S
= 500Ω
R
S
= 15Ω
R
S
100
R
= 50Ω
S
= 100Ω
PIN
. R
IN
8
and C
(which is
AD7622
7
+ t
1000
8
IN
) ~
8
.