EVAL-AD7643CBZ Analog Devices Inc, EVAL-AD7643CBZ Datasheet - Page 17

• Manufacturer Part Number: EVAL-AD7643CBZ
• Description: BOARD EVALUATION FOR AD7643
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7643CBZ

• Number Of Adc's: 1
• Number Of Bits: 18
• Sampling Rate (per Second): 1.25M
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 65mW @ 1.25MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7643
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

TYPICAL CONNECTION DIAGRAM
Figure 23 shows a typical connection diagram for the AD7643.
Different circuitry shown in this diagram is optional and is
discussed in the following sections.
ANALOG INPUTS
Figure 24 shows an equivalent circuit of the input structure of
the AD7643.
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 AD7643 is a true differential structure.
By using this differential input, small signals common to both
inputs are rejected, as shown in Figure 25, 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–
65
60
55
50
45
AGND
1
Figure 25. Analog Input CMRR vs. Frequency
Figure 24. AD7643 Simplified Analog Input
IN
is typically 175 Ω and is a lumped component
INT REF
EXT REF
1
and D
C
10
PIN
IN
and C
2
FREQUENCY (kHz)
, provide ESD protection for the
AVDD
PIN
D
D
and the network formed by the
IN
1
2
. C
100
PIN
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 AD7643 is very high, the
AD7643 can be driven directly by a low impedance source
without gain error. To further improve the noise filtering achieved
by the AD7643’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 23. 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 26.
MULTIPLEXED INPUTS
When using the full 1.25 MSPS throughput in multiplexed
applications for a full-scale step, the RC filter, as shown in
Figure 23, does not settle in the required acquisition time, t
These values are chosen to optimize the best SNR performance
of the AD7643. To use the full 1.25 MSPS throughput in
multiplexed applications, the RC should be adjusted to satisfy t
(which is ~ 8.5 × RC time constant). However, lowering R and C
increases the RC filter bandwidth and allows more noise into the
AD7643, which degrades SNR. To preserve the SNR performance
in these applications using the RC filter shown in Figure 23,
the AD7643 should be run with t
1/(t
7
+ t
Figure 26. THD vs. Analog Input Frequency and Source Resistance
–100
–105
–110
–70
–75
–80
–85
–90
–95
8
) ~ 1.12 MSPS.
1
IN
is typically 12 pF and is mainly the ADC sampling
R
INPUT FREQUENCY (kHz)
S
10
= 500Ω
8
> 350 ns; or approximately
R
S
= 50Ω
100
R
PIN
S
R
= 10Ω
S
. R
= 100Ω
IN
and C
AD7643
1000
IN
8
.
8