參數(shù)資料
型號(hào)： EVAL-AD7623CBZ
廠商： Analog Devices Inc
文件頁數(shù)： 9/28頁
文件大小： 0K
描述： BOARD EVALUATION FOR AD7623
標(biāo)準(zhǔn)包裝： 1
系列： PulSAR®
ADC 的數(shù)量： 1
位數(shù)： 16
采樣率（每秒）： 1.33M
數(shù)據(jù)接口： 串行，并聯(lián)
輸入范圍： ±VREF
在以下條件下的電源（標(biāo)準(zhǔn)）： 50mW @ 1.33MSPS
工作溫度： -40°C ~ 85°C
已用 IC / 零件： AD7623
已供物品： 板
相關(guān)產(chǎn)品： AD7623ASTZ-ND - IC ADC 16BIT 1.33MSPS DFF 48LQFP
AD7623ACPZ-ND - IC ADC 16BIT 1.33MSPS 48LFCSP
AD7623ACPZRL-ND - IC ADC 16BIT 1.33MSPS 48LFCSP
AD7623ASTZRL-ND - IC ADC 16BIT 1.33MSPS DFF 48LQFP
AD7623
Rev. 0 | Page 17 of 28
TYPICAL CONNECTION DIAGRAM
Figure 23 shows a typical connection diagram for the AD7623.
Different circuitry from that shown in this diagram are optional
and are discussed in the Analog Inputs section.
ANALOG INPUTS
Figure 24 shows an equivalent circuit of the input structure of
the AD7623.
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 never exceeds the supply rails by more
than 0.3 V, because this causes the diodes to become forward-
biased and to 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 or U2 supplies are different from AVDD. In such a case, an
input buffer with a short-circuit current limitation can be used
to protect the part.
05574-024
D1
RIN
CIN
D2
IN+ OR IN–
AGND
AVDD
CPIN
Figure 24. AD7623 Simplified Analog Input
The analog inputs of the AD7623 are a true differential
structure. By using this differential input, small signals common
to both inputs are rejected, as shown in Figure 25, representing
the typical CMRR over frequency with internal and external
references.
05574-025
FREQUENCY (kHz)
CMRR
(dB)
45
75
70
65
60
55
50
1
10
100
1000
10000
EXT REF
INT REF
Figure 25. Analog Input CMRR vs. Frequency
During the acquisition phase for ac signals, the impedance of
the analog inputs, IN+ and IN, can be modeled as a parallel
combination of Capacitor CPIN and the network formed by the
series connection of RIN and CIN. CPIN is primarily the pin
capacitance. RIN is typically 350 Ω and is a lumped component
comprised of some serial resistors and the on resistance of the
switches. CIN is typically 12 pF and is primarily the ADC
sampling capacitor. During the conversion phase, when the
switches are opened, the input impedance is limited to CPIN. RIN
and CIN make a one-pole, low-pass filter that has a typical 3 dB
cutoff frequency of 50 MHz, thereby reducing an undesirable
aliasing effect while limiting noise from the inputs.
Since the input impedance of the AD7623 is very high, the
AD7623 can be directly driven by a low impedance source
without gain error. To further improve the noise filtering
achieved by the AD7623 analog input circuit, an external,
one-pole RC filter between the amplifier’s outputs and the ADC
analog inputs can be used, as shown in Figure 23. However,
large source impedances significantly affect the ac performance,
especially total harmonic distortion (THD). 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, as shown in
05574-026
INPUT FREQUENCY (kHz)
THD
(dB)
–100
–60
–65
–70
–75
–80
–85
–90
–95
1
10
100
1k
RS = 500Ω
RS = 50Ω
RS = 100Ω
RS = 10Ω
PDBUF = PDREF = LOW
Figure 26. THD vs. Analog Input Frequency and Source Resistance
DRIVER AMPLIFIER CHOICE
Although the AD7623 is easy to drive, the driver amplifier must
meet the following requirements:
Together, the driver amplifier and the AD7623 analog
input circuit must be able to settle for a full-scale step of
the capacitor array at a 16-bit level (0.0015%). In the
amplifier data sheet, settling at 0.1% to 0.01% is more
commonly specified. This could differ significantly from
the settling time at a 16-bit level and should be verified
prior to driver selection. The AD8021 op amp, which
combines ultralow noise and high gain bandwidth, meets
this settling time requirement even when used with gains
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 AD7623. The noise coming from
the driver is filtered by the AD7623 analog input circuit
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