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參數資料
| 型號： | AD7725 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 16-Bit 900 kSPS ADC with a Programmable Postprocessor |
| 中文描述： | 16位900 ksps模數轉換器的可編程后處理器 |
| 文件頁數： | 17/28頁 |
| 文件大?。?/td> | 442K |
| 代理商： | AD7725 |
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REV. A
AD7725
–17–
220pF
AD7725
V
IN
(+)
V
IN
(–)
27
R
IN
390
10k
10nF
220nF
1 F
27
220
AIN = 2V
BIASED
ABOUT
GROUND
REF2
REF1
R
FB
220
R
SOURCE
20k
220
220
50
Figure 14. Single-Ended-to-Differential Input Circuit for Bipolar
Mode Operation (Analog Input Biased about Ground)
Applying the Reference
The AD7725 can operate with either an external reference or
with its on-chip 2.5 V reference. A block diagram of the internal
reference circuit is shown in Figure 15. The internal reference
circuitry includes an on-chip 2.5 V band gap reference and a
reference buffer circuit. The internal 2.5 V reference voltage is
connected to the REF1 pin through a 3 k
resistor. It is buffered
to drive the analog modulator’s switched capacitor DAC (REF2)
as shown in Figure 15. When using the internal reference, a 1
μ
F
capacitor is required between REF1 and AGND to decouple the
band gap noise. If the internal reference is required to bias
external circuits, an external precision op amp should be used to
buffer REF1.
REF1
AD7725
2.5
REFERENCE
1 F
REF2
1V
COMPARATOR
REFERENCE
BUFFER
3k
SWITCHED-CAPACITOR
DAC REFERENCE
220nF
10nF
Figure 15. Reference Circuit Block Diagram
Where gain error or gain drift requires the use of an external
reference, this can be applied directly to the REF2 pin. In this
case, the reference buffer in Figure 15 can be turned off by
grounding the REF1 pin. The AD7725 will accept an external
reference voltage between 1.2 V and 3.15 V.
Where the output common-mode range of the amplifier driving
the inputs is restricted, the full-scale input signal span can be
reduced by applying a lower than 2.5 V reference. For example,
a 1.25 V reference would make the bipolar (differential) input
range
±
1 V but would degrade SNR.
In all cases, since the REF2 voltage connects to the analog
modulator, a 220 nF and 10 nF capacitor must connect directly
from REF2 to AGND. The external capacitors provide the
charge required for the dynamic load presented at the REF2 pin
(see Figure 16).
A
B
4pF
CLKIN
A
A
B
B
AD7725
B
A
4pF
SWITCHED-CAPACITOR
DAC REFERENCE
10nF
REF2
220nF
Figure 16. REF2 Equivalent Input Circuit
The AD780 is ideal to use as an external reference with the
AD7725. Figure 17 shows a suggested connection diagram.
Grounding Pin 8 on the AD780 selects the 3 V output mode.
AD7725
REF1
8
7
6
5
1
2
3
4
NC = NO CONNECT
O/P
SELECT
NC
NC
+V
IN
TEMP
GND
V
OUT
TRIM
AD780
5V
22nF
1 F
REF2
220nF
10nF
22 F
2.5V
Figure 17. External Reference Circuit Connection
Clock Generation
The AD7725 has an on-chip oscillator circuit to allow a crystal
or an external clock signal to generate the master clock for
the ADC. In both cases, the clock input has to be continual;
once power is applied to the AD7725, it has to be continually
clocked. The connection diagram for use with a crystal is shown
in Figure 18. Consult the manufacturer’s recommendation for
the load capacitors. To enable the oscillator circuit on the
AD7725, XTAL_OFF should be tied low.
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