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參數資料
| 型號： | AD7671 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 16-Bit 1 MSPS SAR Unipolar ADC with Ref |
| 中文描述： | 16位1 MSPS的特區單極型ADC檔 |
| 文件頁數： | 14/23頁 |
| 文件大小： | 336K |
| 代理商： | AD7671 |
REV. PrA
PRELIMINARY TECHNICAL DATA
–14–
AD7667
different from AVDD. In such case, an input buffer with a
short circuit current limitation can be used to protect the
part.
T his analog input structure allows the sampling of the
differential signal between IN and INGND. Unlike other
converters, the INGND input is sampled at the same time as
the IN input. By using this differential input, small signals
common to both inputs are rejected, as shown in Figure 7,
which represents the typical CMRR over frequency. For in-
stance, by using INGND to sense a remote signal ground,
difference of ground potentials between the sensor and the
local ADC ground are eliminated.
-160
-140
-120
-100
-80
-60
-40
-20
0
1
TO BE
SUPPLIED
Figure 7. Analog Input CMR vs. Frequency
During the acquisition phase, the impedance of the analog
input IN can be modeled as a parallel combination of ca-
pacitor C1 and the network formed by the series connection
of R1 and C2. Capacitor C1 is primarily the pin capacitance.
T he resistor R1 is typically 183
nent made up of some serial resistors and the on resistance of
the switches. T he capacitor C2 is typically 60 pF and is mainly
the ADC sampling capacitor. During the conversion phase, where
the switches are opened, the input impedance is limited to C1. T he
R1, C2 makes a one-pole low-pass filter that reduces undesir-
able aliasing effect and limits the noise.
When the source impedance of the driving circuit is low,
the AD7667 can be driven directly. L arge source imped-
ances will significantly affect the ac performances,
especially the total harmonic distortion. T he maximum
source impedance depends on the amount of total harmonic
distortion (T HD) that can be tolerated. T he T HD degrades
in function of the source impedance and the maximum input
frequency as shown in Figure
T BD.
and is a lumped compo-
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-140
-120
-100
-80
-60
-40
-20
0
1
TO BE
SUPPLIED
Figure 8. THD vs. Analog Input Frequency and
Source Resistance
D river Amplifier C hoice
Although the AD7667 is easy to drive, the driver amplifier
needs to meet at least the following requirements:
T he driver amplifier and the AD7667 analog input circuit
must be able together to settle for a full-scale step the capaci-
tor array at a 16-bit level (0.0015%). In the amplifier’s data
sheet, the settling at 0.1% to 0.01% is more commonly speci-
fied. It could significantly differ from the settling time at 16
bit level and it should therefore be verified prior to the
driver selection. T he tiny op amp AD8021, which com-
bines ultralow noise and a high-gain bandwidth, meets
this settling time requirement even when used with high
gain up to 13.
T he 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 AD7667. T he noise
coming from the driver is filtered by the AD7667 analog
input circuit one-pole low-pass filter made by R1 and C2
or the external filter if any is used. T he SNR degredation
due to the amplifier is:
f-3dB
(
N eN
)
2
2
784 +
(
)
28
SNR
LOSS
= 20 LOG
where
f
–3dB
is the –3 dB input bandwidth of the AD7667 in MHz
(14.5) or the cutoff frequency of the input filter if
any used.
N
is the noise gain of the amplifier (1 if in buffer
configuration).
e
N
is the equivalent input noise voltage of the op amp
in
(H z)
1/2
.
For instance, a driver like the AD8021, with an equivalent
input noise of 2 nV/
Hz and configured as a buffer, thus with
a noise gain of 1, the SNR degrades by only 0.13 dB with the
filter used in figure 5.
T he driver needs to have a T HD performance suitable to that
of the AD7667.
T he AD8021 meets these requirements and is usually appro-
priate for almost all applications. T he AD8021 needs an
external compensation capacitor of 10 pF. T his capacitor
should have good linearity as an NPO ceramic or mica type.
T he AD8022 could also be used where dual version is needed
and gain of 1 is used.
nV /
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