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| 型號： | AD7789 |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | ADC |
| 英文描述： | Low Power, 16-/24-Bit Sigma-Delta ADC |
| 中文描述： | 低功耗，16-/24-BitΣ-Δ模數(shù)轉(zhuǎn)換器 |
| 文件頁數(shù)： | 17/20頁 |
| 文件大小： | 288K |
| 代理商： | AD7789 |
AD7788/AD7789
CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7788/AD7789 has one differential analog input channel
that is connected to the modulator, i.e., the input is unbuffered.
Note that this unbuffered input path provides a dynamic load to
the driving source. Therefore, resistor/capacitor combinations
on the input pins can cause dc gain errors, depending on the
output impedance of the source that is driving the ADC input.
Table 13 shows the allowable external resistance/capacitance
values such that no gain error at the 16-bit level is introduced
(AD7788), while Table 14 shows the allowable external resis-
tance/capacitance values such that no gain error at the 20-bit
level is introduced (AD7789).
Rev. 0 | Page 17 of 20
Table 13. External R-C Combination for No 16-Bit Gain
Error (AD7788)
C (pF)
R ()
50
22.8K
100
13.1K
500
3.3K
1000
1.8K
5000
360
Table 14. External R-C Combination for No 20-Bit Gain
Error (AD7789)
C (pF)
R ()
50
16.7K
100
9.6K
500
2.2K
1000
1.1K
5000
160
The absolute input voltage includes the range between GND –
30 mV and V
DD
+ 30 mV. The negative absolute input voltage
limit does allow the possibility of monitoring small true bipolar
signals with respect to GND.
The absolute input voltage includes the range between GND –
30 mV and V
DD
+ 30 mV. The negative absolute input voltage
limit does allow the possibility of monitoring small true bipolar
signals with respect to GND.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7788/AD7789 can accept either
unipolar or bipolar input voltage ranges. A bipolar input range
does not imply that the part can tolerate large negative voltages
with respect to system GND. Unipolar and bipolar signals on
the AIN(+) input are referenced to the voltage on the AIN(–)
input. For example, if AIN(–) is 2.5 V and the ADC is config-
ured for unipolar mode, the input voltage range on the AIN(+)
pin is 2.5 V to 5 V. If the ADC is configured for bipolar mode,
the analog input range on the AIN(+) input is 0 V to 5 V. The
bipolar/unipolar option is chosen by programming the B/U bit
in the mode register.
DATA OUTPUT CODING
When the ADC is configured for unipolar operation, the output
code is natural (straight) binary with a zero differential input
voltage resulting in a code of 00...00, a midscale voltage result-
ing in a code of 100...000, and a full-scale input voltage resulting
in a code of 111...111. The output code for any analog input
voltage can be represented as
Code
= 2
N
× (
AIN
/
V
REF
)
When the ADC is configured for bipolar operation, the output
code is offset binary with a negative full-scale voltage resulting
in a code of 000...000, a zero differential input voltage resulting
in a code of 100...000, and a positive full-scale input voltage
resulting in a code of 111...111. The output code for any analog
input voltage can be represented as
Code
= 2
N
– 1
× [(
AIN
/
V
REF
) + 1]
where
AIN
is the analog input voltage and
N
= 16 for the
AD7788 and 24 for the AD7789.
REFERENCE INPUT
The AD7788/AD7789 has a fully differential input capability for
the channel. The common-mode range for these differential
inputs is from GND to V
DD
. The reference input is unbuffered
and, therefore, excessive R-C source impedances will introduce
gain errors. The reference voltage REFIN (REFIN(+) –
REFIN(–)) is 2.5 V nominal, but the AD7788/AD7789 is func-
tional with reference voltages from 0.1 V to V
DD
. In applications
where the excitation (voltage or current) for the transducer on
the analog input also drives the reference voltage for the part,
the effect of the low frequency noise in the excitation source
will be removed because the application is ratiometric. If the
AD7788/AD7789 is used in a nonratiometric application, a low
noise reference should be used.
Recommended 2.5 V reference voltage sources for the AD7788/
AD7789 include the ADR381 and ADR391, which are low noise,
low power references. If the analog circuitry uses a 2.5 V power
supply, the reference voltage source will require some head-
room. In this case, a 2.048 V reference such as the ADR380 or
ADR390 can be used. Again, these are low power, low noise
references. Also note that the reference inputs provide a high
impedance, dynamic load. Because the input impedance of each
reference input is dynamic, resistor/capacitor combinations on
these inputs can cause dc gain errors, depending on the output
impedance of the source that is driving the reference inputs.
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