AD7621 Analog Devices, AD7621 Datasheet - Page 17

• Manufacturer Part Number: AD7621
• Description: 16-Bit, 2 LSB INL, 3 MSPS PulSAR® ADC
• Manufacturer: Analog Devices

Specifications of AD7621

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 3MSPS
• 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 23 shows a typical connection diagram for the AD7621.
Different circuitry from that shown in this diagram are optional
and are discussed below.
ANALOG INPUTS
Figure 24 shows an equivalent circuit of the input structure of
the AD7621.
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 as this causes the diodes to become forward-biased
and 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 AD7621 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
IN+ OR IN–
75
70
65
60
55
50
45
AGND
1
Figure 25. Analog Input CMRR vs. Frequency
Figure 24. AD7621 Simplified Analog Input.
IN
is typically 350 Ω and is a lumped component
1
C
and D
PIN
10
IN
EXT REF
and C
2
FREQUENCY (kHz)
AVDD
, provide ESD protection for the
PIN
D
D
IN
and the network formed by the
1
2
. C
100
INT REF
PIN
is primarily the pin
R
IN
1k
C
IN
10k
Rev. 0 | Page 17 of 32
comprised of some serial resistors and the on resistance of the
switches. C
capacitor. During the conversion phase, when the switches are
opened, the input impedance is limited to C
make a one-pole, low-pass filter that has a typical −3 dB cutoff
frequency of 50 MHz, thereby reducing an undesirable aliasing
effect while limiting noise from the inputs.
Since the input impedance of the AD7621 is very high, the
AD7621 can be directly driven by a low impedance source
without gain error. To further improve the noise filtering
achieved by the AD7621 analog input circuit, an external, one-
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 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.
DRIVER AMPLIFIER CHOICE
Although the AD7621 is easy to drive, the driver amplifier
needs to meet the following requirements:
•
•
Together, the driver amplifier and the AD7621 analog
input circuit must be able to settle for a full-scale step of
the capacitor array at a 16-bit level (0.0015%). In the
amplifier data sheet, settling at 0.1% to 0.01% is more
commonly specified. This could differ significantly from
the settling time at a 16-bit level and should be verified
prior to driver selection. The
combines ultralow noise and high gain bandwidth, meets
this settling time requirement even when used with gains
up to 13.
The noise generated by the driver amplifier needs to be
kept as low as possible in order to preserve the SNR and
transition noise performance of the AD7621. The noise
Figure 26. THD vs. Analog Input Frequency and Source Resistance
–100
–105
–60
–65
–70
–75
–80
–85
–90
–95
1
PDBUF = PDREF = LOW
IN
is typically 12 pF and is mainly the ADC sampling
INPUT FREQUENCY (kHz)
10
AD8021
R
S
= 50Ω
100
R
S
op amp, which
PIN
= 100Ω
R
S
. R
R
= 10Ω
S
IN
= 500Ω
and C
AD7621
1k
IN