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| 型號： | AD7952BSTZRL |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 14/32頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC ADC 14BIT DIFF 1MSPS 48-LQFP |
| 標準包裝： | 2,000 |
| 系列： | PulSAR® |
| 位數(shù)： | 14 |
| 采樣率（每秒）： | 1M |
| 數(shù)據(jù)接口： | 并聯(lián) |
| 轉換器數(shù)目： | 1 |
| 功率耗散（最大）： | 260mW |
| 電壓電源： | 模擬和數(shù)字，雙 ± |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 48-LQFP |
| 供應商設備封裝： | 48-LQFP（7x7） |
| 包裝： | 帶卷 (TR) |
| 輸入數(shù)目和類型： | 1 個差分，雙極 |
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Data Sheet
AD7952
Rev. A | Page 21 of 32
DRIVER AMPLIFIER CHOICE
Although the AD7952 is easy to drive, the driver amplifier must
meet the following requirements:
For multichannel, multiplexed applications, the driver
amplifier and the AD7952 analog input circuit must be
able to settle for a full-scale step of the capacitor array at a
14-bit level (0.006%). For the amplifier, settling at 0.1% to
0.01% is more commonly specified. This differs significantly
from the settling time at a 14-bit level and should be
verified prior to driver selection. The AD8021 op amp com-
bines ultralow noise and high gain bandwidth and meets
this settling time requirement even when used with gains
of up to 13.
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 AD7952. The noise coming from
the driver is filtered by the external 1-pole, low-pass filter,
as shown in Figure 27. The SNR degradation due to the
amplifier is
2
3dB
2
3dB
2
)
(
f
2
)
(
f
2
log
20
N
NADC
LOSS
Ne
V
SNR
where:
VNADC is the noise of the ADC, which is:
20
10
2
SNR
INp-p
NADC
V
f–3dB is the cutoff frequency of the input filter (3.9 MHz).
N is the noise factor of the amplifier (1 in the buffer
configuration).
eN+ and eN are the equivalent input voltage noise densities
of the op amps connected to IN+ and IN, in nV/√Hz.
When the resistances used around the amplifiers are small,
this approximation can be used. If larger resistances are
used, their noise contributions should also be root-sum
squared.
The driver needs to have a THD performance suitable to
that of the AD7952. Figure 15 shows the THD vs. frequency
that the driver should exceed.
The AD8021 meets these requirements and is appropriate for
almost all applications. The AD8021 needs a 10 pF external
compensation capacitor that should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
+1 gain arrangement is recommended and helps to obtain the
best SNR.
The AD8022 can also be used when a dual version is needed
and a gain of 1 is present. The AD829 is an alternative in
applications where high frequency performance (above 100 kHz)
is not required. In applications with a gain of 1, an 82 pF
compensation capacitor is required. The AD8610 is an option
when low bias current is needed in low frequency applications.
Because the AD7952 uses a large geometry, high voltage input
switch, the best linearity performance is obtained when using
the amplifier at its maximum full power bandwidth. Gaining
the amplifier to make use of the more dynamic range of the
ADC results in increased linearity errors. For applications
requiring more resolution, the use of an additional amplifier
with gain should precede a unity follower driving the AD7952.
See Table 8 for a list of recommended op amps.
Table 8. Recommended Driver Amplifiers
Amplifier
Typical Application
±15 V supplies, very low noise, low frequency
±12 V supplies, very low noise, high frequency
±12 V supplies, very low noise, high
frequency, dual
±12 V supplies, low noise, high frequency,
single-ended-to-differential driver
±13 V supplies, low bias current, low
frequency, single/dual
Single-to-Differential Driver
For single-ended sources, a single-to-differential driver, such
as the ADA4922-1, can be used because the AD7952 needs to
be driven differentially. The 1-pole filter using R = 15 Ω and
C = 2.7 nF provides a corner frequency of 3.9 MHz.
0
6589
-047
ANALOG
INPUT
IN+
IN–
AD7952
REF
10F
15
100nF
2.7nF
U2
R1
R2
ADA4922-1
OUT+
VCC
VEE
OUT–
IN
REF
RF
RG
Figure 30. Single-to-Differential Driver Using the ADA4922-1
For unipolar 5 V and 10 V input ranges, the internal (or
external) reference source can be used to level shift U2 for
the correct input span. If using an external reference, the
values for R1/R2 can be lowered to reduce resistive Johnson
noise (1.29E 10 × √R). For the bipolar ±5 V and ±10 V input
ranges, the reference connection is not required because the
common-mode voltage is 0 V. See Table 9 for R1/R2 for the
different input ranges.
Table 9. R1/R2 Configuration
Input Range (V)
R1 (Ω)
R2 (Ω)
Common-ModeVoltage (V)
5
2.5 k
2.5
10
2.5 k
Open
5
±5, ±10
100
0
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