AD7652 Analog Devices, AD7652 Datasheet - Page 17

• Manufacturer Part Number: AD7652
• Manufacturer: Analog Devices

Specifications of AD7652

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 500kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

TYPICAL CONNECTION DIAGRAM
Figure 22 shows a typical connection diagram for the AD7652.
Analog Input
Figure 23
the AD7652.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs IN and INGND. Care must be taken to ensure
that the analog input signal never exceeds the supply rails by
more than 0.3 V. This will cause these 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) 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.
This analog input structure allows the sampling of the
differential signal between IN and INGND. Unlike other
converters, INGND is sampled at the same time as IN. By using
this differential input, small signals common to both inputs are
rejected. For instance, by using INGND to sense a remote signal
ground, ground potential differences between the sensor and
the local ADC ground are eliminated.
During the acquisition phase, the impedance of the analog input
IN can be modeled as a parallel combination of capacitor C1
and the network formed by the series connection of R1 and C2.
C1 is primarily the pin capacitance. R1 is typically 168 Ω and is
a lumped component made up of some serial resistors and the
on resistance of the switches. C2 is typically 60 pF and is mainly
the ADC sampling capacitor. During the conversion phase,
when the switches are opened, the input impedance is limited to
C1. R1 and C2 make a 1-pole low-pass filter that reduces
undesirable aliasing effect and limits the noise.
When the source impedance of the driving circuit is low, the
AD7652 can be driven directly. Large source impedances will
significantly affect the ac performance, especially total
harmonic distortion.
OR INGND
AGND
IN
shows an equivalent circuit of the input structure of
Figure 23. Equivalent Analog Input Circuit
C1
AVDD
D1
D2
R1
C2
02965-0-008
Rev. 0 | Page 17 of 28
Driver Amplifier Choice
Although the AD7652 is easy to drive, the driver amplifier needs
to meet the following requirements:
•
•
•
The
almost all applications. The
compensation capacitor that should have good linearity as an
NPO ceramic or mica type.
The
gain of +1 is present. The
applications where high frequency (above 100 kHz)
performance is not required. In gain of 1 applications, it requires
an 82 pF compensation capacitor. The
when low bias current is needed in low frequency applications.
AD8021
AD8022
The driver amplifier and the AD7652 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’s data
sheet, settling at 0.1% to 0.01% is more commonly speci-
fied. This could differ significantly from the settling time at
a 16-bit level and should be verified prior to driver
selection. The tiny op amp AD8021, which 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 AD7652. The noise
coming from the driver is filtered by the AD7652 analog
input circuit 1-pole low-pass filter made by R1 and C2 or
by the external filter, if one is used.
The driver needs to have a THD performance suitable to
that of the AD7652.
meets these requirements and is appropriate for
could also be used if a dual version is needed and
AD829
AD8021
is an alternative in
needs a 10 pF external
AD8610
is an option
AD7652