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参数资料
| 型号： | EVAL-AD7722CBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 8/24页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR AD7722CBZ |
| 标准包装： | 1 |
| ADC 的数量： | 1 |
| 位数： | 16 |
| 采样率（每秒）： | 220k |
| 数据接口： | 串行，并联 |
| 输入范围： | ±VREF/2 |
| 在以下条件下的电源（标准）： | 375mW @ 220kSPS |
| 工作温度： | -40°C ~ 85°C |
| 已用 IC / 零件： | AD7722 |
| 已供物品： | 板，CD |
REV. B
–16–
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.
R
100
C
2.7nF
AD7722
VIN(+)
VIN(–)
C
2.7nF
C
2.7nF
R
100
Figure 15. Differential Input with Antialiasing
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 (f3 dB) of the
antialias filter is given by Equation 1, and the attenuation of the
filter is given by Equation 2.
f
RC
dB
3
1
6
=
(1)
Attenuation
log
f
f dB
=+
20
1 / 1
3
2
(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
and
2.7 nF shown in Figure 15, the –3 dB cutoff frequency (f3 dB)
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 k
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
required to bias external circuits, use an external precision op
amp to buffer REF1.
24
3k
AD7722
REFERENCE
BUFFER
22
1V
2.5V
REFERENCE
SWITCHED-CAP
DAC REF
REF1
REF2
COMPARATOR
100nF
Figure 16. Reference Circuit Block Diagram
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).
ΦA
ΦB
24
4pF
ΦA ΦB ΦA ΦB
CLKIN
REF2
AD7722
ΦA
4pF
SWITCHED-CAP
DAC REF
100nF
Figure 17. REF2 Equivalent Input Circuit
The AD780 is ideal to use as an external reference with the
AD7722. Figure 18 shows a suggested connection diagram.
AD780
1
2
3
4
8
7
6
5
NC
+VIN
TEMP
GND
O/P
SELECT
NC
VOUT
TRIM
22nF
1 F
24
REF2
AD7722
22 F
100nF
22
REF1
5V
Figure 18. External Reference Circuit Connection
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