AD7722ASZ Analog Devices Inc, AD7722ASZ Datasheet - Page 16

• Manufacturer Part Number: AD7722ASZ
• Description: ADC Single Delta-Sigma 195KSPS 16-Bit Parallel/Serial 44-Pin MQFP
• Manufacturer: Analog Devices Inc

Specifications of AD7722ASZ

• Package: 44MQFP
• Resolution: 16 Bit
• Sampling Rate: 195 KSPS
• Architecture: Delta-Sigma
• Number Of Analog Inputs: 1
• Digital Interface Type: Parallel|Serial (3-Wire)
• Input Type: Voltage
• Polarity Of Input Voltage: Unipolar|Bipolar
• Number Of Bits: 16
• Sampling Rate (per Second): 220k
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 375mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 44-QFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7722
Even though the voltage on the input sampling capacitors may not
have enough time to settle to the accuracy indicated by the resolu-
tion of the AD7722, as long as the sampling capacitor charging
follows the exponential curve of RC circuits, only the gain
accuracy suffers if the input capacitor is switched away too early.
An alternative circuit configuration for driving the differential
inputs to the AD7722 is shown in Figure 15.
A capacitor between the two input pins sources or sinks charge
to allow most of the charge that is needed by one input to be
effectively supplied by the other input. This minimizes undesir-
able charge transfer from the analog inputs to and from ground.
The series resistor isolates the operational amplifier from the
current spikes created during the sampling process and provides
a pole for antialiasing. The –3 dB cutoff frequency (f
antialias filter is given by Equation 1, and the attenuation of the
filter is given by Equation 2.
The choice of the filter cutoff frequency will depend on the
amount of roll-off that is acceptable in the pass band of the
digital filter and the required attenuation at the first image
frequency. For example, when operating the AD7722 with a
12.5 MHz clock, with the typical values of R and C of 100 Ω and
2.7 nF shown in Figure 15, the –3 dB cutoff frequency (f
creates less than 1 dB of in-band (90.625 kHz) roll-off and
provides about 36 dB attenuation at the first image frequency.
The capacitors used for the input antialiasing circuit must have
low dielectric absorption to avoid distortion. Film capacitors such
as polypropylene, polystyrene, or polycarbonate are suitable. If
ceramic capacitors are used, they must have NP0 dielectric.
Applying the Reference
The reference circuitry used in the AD7722 includes an on-chip
2.5 V band gap reference and a reference buffer circuit. The block
diagram of the reference circuit is shown in Figure 16. The inter-
nal reference voltage is connected to REF1 through a 3 kΩ resistor
and is internally buffered to drive the analog modulator’s switched
cap DAC (REF2). When using the internal reference, connect
100 nF between REF1 and AGND. If the internal reference is
Attenuation
f
3
Figure 15. Differential Input with Antialiasing
dB
=
6
1
RC
=
100
100
20
R
R
log




1 / 1
+
C
2.7nF
C
2.7nF
C
2.7nF



f
3
f
dB



2
V
V
IN




IN
(–)
(+)
AD7722
3 dB
) of the
3 dB
)
(1)
(2)
–16–
required to bias external circuits, use an external precision op
amp to buffer REF1.
The AD7722 can operate with its internal reference, or an
external reference can be applied in two ways. An external
reference can be connected to REF1, overdriving the internal
reference. However, there will be an error introduced due to the
offset of the internal buffer amplifier. For the lowest system gain
errors when using an external reference, REF1 is grounded
(disabling the internal buffer) and the external reference is
connected to REF2.
In all cases, since the REF2 voltage connects to the analog
modulator, a 100 nF capacitor must connect directly from
REF2 to AGND. The external capacitor provides the charge
required for the dynamic load presented at the REF2 pin
(Figure 17).
The AD780 is ideal to use as an external reference with the
AD7722. Figure 18 shows a suggested connection diagram.
100nF
1 F
REF1
REF2
Figure 18. External Reference Circuit Connection
5V
22nF
Figure 16. Reference Circuit Block Diagram
100nF
22
24
Figure 17. REF2 Equivalent Input Circuit
REF2
1V
24
1
2
3
4
NC
+V
TEMP
GND
IN
AD780
CLKIN
3k
SELECT
COMPARATOR
TRIM
V
OUT
O/P
NC
REFERENCE
BUFFER
8
7
6
5
Φ
REFERENCE
A
Φ
Φ
B
A
Φ
2.5V
B
22 F
Φ
4pF
4pF
100nF
A
Φ
SWITCHED-CAP
DAC REF
B
Φ
Φ
AD7722
B
A
AD7722
24
SWITCHED-CAP
DAC REF
22
REF2
REF1
AD7722
REV. B