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參數(shù)資料
| 型號： | ADA4941-1 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 16-Bit, +/-0.65 LSB INL, 500 kSPS PulSAR Differential ADC in MSOP/QFN |
| 中文描述： | 16位/ -0.65 LSB INL和500 kSPS的PulSAR系列差分ADC，采用MSOP / QFN封裝 |
| 文件頁數(shù)： | 14/24頁 |
| 文件大?。?/td> | 449K |
| 代理商： | ADA4941-1 |
AD7693
Preliminary Technical Data
ANALOG INPUTS
Figure 27 shows an equivalent circuit of the input structure of
the AD7693.
Rev. PrB | Page 14 of 24
When the source impedance of the driving circuit is low, the
AD7693 can be driven directly. Large source impedances
significantly affect the ac performance, especially total
harmonic distortion (THD). The dc performances are less
sensitive to the input impedance. The maximum source
impedance depends on the amount of THD that can be
tolerated. The THD degrades as a function of the source
impedance and the maximum input frequency.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs, IN+ and IN. Care must be taken to ensure that
the analog input signal does not exceed the supply rails by more
than 0.3 V because this causes the diodes to become forward
biased and start conducting current. These diodes can handle a
forward-biased current of 130 mA maximum. For instance,
these conditions could eventually occur when the input buffer’s
(U1) supplies are different from VDD. In such a case, for
example, an input buffer with a short-circuit, the current
limitation can be used to protect the part.
DRIVER AMPLIFIER CHOICE
Although the AD7693 is easy to drive, the driver amplifier must
meet the following requirements:
The noise generated by the driver amplifier needs to be
kept as low as possible to preserve the SNR and transition
noise performance of the AD7693. The noise coming from
the driver is filtered by the AD7693 analog input circuit’s
1-pole, low-pass filter made by RIN and CIN or by the
external filter, if one is used. Because the typical noise of
the AD7693 is 56 μV rms, the SNR degradation due to the
amplifier is
C
IN
R
IN
D1
D2
C
PIN
IN+
OR IN–
GND
VDD
-
+
+
=
2
dB
3
2
dB
3
2
)
(
2
π
)
(
2
π
56
56
log
20
N
N
LOSS
Ne
f
Ne
f
SNR
Figure 27. Equivalent Analog Input Circuit
The analog input structure allows the sampling of the true
differential signal between IN+ and IN. By using these
differential inputs, signals common to both inputs are rejected.
where:
f
3 dB
is the input bandwidth in megahertz of the AD7693
(9 MHz) or the cutoff frequency of the input filter, if one is
used.
N
is the noise gain of the amplifier (for example, 1 in
buffer configuration).
e
N
is the equivalent input noise voltage of the op amp, in
nV/√Hz.
90
40
1
10000
FREQUENCY (kHz)
C
-
10
100
1000
85
80
75
70
65
60
55
50
45
V
REF
= VDD = 5V
For ac applications, the driver should have a THD
performance commensurate with the AD7693.
For multichannel multiplexed applications, the driver
amplifier and the AD7693 analog input circuit must settle
for a full-scale step onto the capacitor array at an 16-bit
level (0.0015%, 15 ppm). In the amplifier’s data sheet,
settling at 0.1% to 0.01% is more commonly specified. This
could differ significantly from the settling time at an 16-bit
level and should be verified prior to driver selection.
Figure 28. Analog Input CMRR vs. Frequency
During the acquisition phase, the impedance of the analog
inputs (IN+ and IN) can be modeled as a parallel combination
of the capacitor, C
PIN
, and the network formed by the series
connection of R
IN
and C
IN
. C
PIN
is primarily the pin capacitance.
R
IN
is typically 600 Ω and is a lumped component made up of
serial resistors and the on resistance of the switches. C
IN
is
typically 30 pF and is mainly the ADC sampling capacitor.
Table 8. Recommended Driver Amplifiers
Amplifier
Typical Application
ADA4941-1
Very low noise, low power single to differential
ADA4841-x
Very low noise, small, and low power
AD8655
5 V single supply, low noise
AD8021
Very low noise and high frequency
AD8022
Low noise and high frequency
OP184
Low power, low noise, and low frequency
AD8605
,
AD8615
5 V single supply, low power
During the conversion phase, where the switches are opened,
the input impedance is limited to C
PIN
. R
IN
and C
IN
make a 1-
pole, low-pass filter that reduces undesirable aliasing effects and
limits the noise.
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