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參數(shù)資料
| 型號： | AD7660ASTZRL |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 4/20頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC ADC 16BIT UNIPOLAR 48LQFP |
| 標(biāo)準(zhǔn)包裝： | 2,000 |
| 系列： | PulSAR® |
| 位數(shù)： | 16 |
| 采樣率（每秒）： | 100k |
| 數(shù)據(jù)接口： | 串行，并聯(lián) |
| 轉(zhuǎn)換器數(shù)目： | 1 |
| 功率耗散（最大）： | 25mW |
| 電壓電源： | 模擬和數(shù)字 |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 48-LQFP |
| 供應(yīng)商設(shè)備封裝： | 48-LQFP（7x7） |
| 包裝： | 帶卷 (TR) |
| 輸入數(shù)目和類型： | 2 個偽差分，單極 |
| 配用： | EVAL-AD7660CBZ-ND - BOARD EVALUATION FOR AD7660 |
REV. D
AD7660
–12–
Analog Input
Figure 6 shows an equivalent circuit of the input structure of the
AD7660.
C2
R1
D1
D2
C1
IN
OR INGND
AGND
AVDD
Figure 6. Equivalent Analog Input Circuit
The two diodes D1 and D2 provide ESD protection for the
analog inputs IN and INGND. Care must be taken to ensure that
the analog input signal never exceeds the supply rails by more than
0.3 V. This will cause these diodes to become forward-biased and
start conducting current. These diodes can handle a forward-
biased current of 100 mA maximum. For instance, these
conditions could eventually occur when the input buffer’s (U1)
supplies are different from AVDD. In such cases, an input buffer
with a short circuit current limitation can be used to protect the part.
This 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 instance, by using INGND to sense
a remote signal ground, difference of ground potentials between
the sensor and the local ADC ground is eliminated.
FREQUENCY – Hz
CMRR
–
dB
45
75
10k
10M
1k
1M
80
65
100k
55
85
70
60
50
40
Figure 7. Analog Input CMRR vs. Frequency
During the acquisition phase, the impedance of the analog input
IN can be modeled as a parallel combination of capacitor C1 and
the network formed by the series connection of R1 and C2.
Capacitor C1 is primarily the pin capacitance. The resistor R1 is
typically 3242
W and is a lumped component made up of some
serial resistors and the on resistance of the switches. The 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. It has to be noted that the input
impedance of the AD7660, unlike other SAR ADCs, is not a pure
capacitance and thus, inherently reduces the kickback transient at
the beginning of the acquisition phase. The R1, C2 makes a one-
pole low-pass filter with a typical cutoff frequency of 820 kHz that
reduces undesirable aliasing effect and limits the noise.
When the source impedance of the driving circuit is low, the
AD7660 can be driven directly. Large source impedances will
significantly affect the ac performances, especially the total
harmonic distortion (THD). The maximum source impedance
depends on the amount of THD that can be tolerated. The
THD degrades in function of the source impedance and the
maximum input frequency as shown in Figure 8.
INPUT FREQUENCY – kHz
–100
1
100
10
THD
–
dB
–95
–90
–85
–80
–75
–70
RS = 500
RS = 100
RS = 50
RS = 20
Figure 8. THD vs. Analog Input Frequency and
Source Resistance
Driver Amplifier Choice
Although the AD7660 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
The driver amplifier and the AD7660 analog input circuit
must be able, together, to settle for a full-scale step of
the capacitor array at a 16-bit level (0.0015%). For
instance, operation at the maximum throughput of 100 kSPS
requires a minimum gain bandwidth product of 5 MHz.
The 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 AD7660. The noise
coming from the driver is filtered by the AD7660 analog
input circuit one-pole low-pass filter made by R1 and C2.
For instance, a driver with an equivalent input noise of
4 nV/
÷Hz like the OP184 and configured as a buffer, thus
with a noise gain of +1, degrades the SNR by only 0.1 dB.
The driver needs to have a THD performance suitable to
that of the AD7660. TPC 8 gives the THD versus frequency
that the driver should preferably exceed.
The SNR degradation due to the amplifier is:
SNR
fN e
LOSS
dB
N
=
+
20
28
784
2
3
2
log
()
–
p
where:
f–3 dB is the –3 dB input bandwidth in MHz of the AD7660
(0.82 MHz) or the cutoff frequency of the input filter if any
are used.
N is the noise factor of the amplifier (1 if in buffer configuration).
eN is the equivalent input noise voltage of the op amp in
nV/
÷Hz.
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