input sampling capacitor, C
參數(shù)資料
型號(hào)： AD7715ARZ-3REEL
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 13/40頁(yè)
文件大小： 0K
描述： IC ADC 16BIT SIGMA-DELTA 16SOIC
標(biāo)準(zhǔn)包裝： 1,000
位數(shù)： 16
采樣率（每秒）： 500
數(shù)據(jù)接口： DSP，MICROWIRE?，QSPI?，串行，SPI?
轉(zhuǎn)換器數(shù)目： 1
功率耗散（最大）： 9.5mW
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： 16-SOIC（0.295"，7.50mm 寬）
供應(yīng)商設(shè)備封裝： 16-SOIC W
包裝： 帶卷 (TR)
輸入數(shù)目和類型： 1 個(gè)差分，單極；1 個(gè)差分，雙極
配用： EVAL-AD7715-3EBZ-ND - BOARD EVALUATION FOR AD7715
AD7715
Rev. D | Page 20 of 40
In unbuffered mode, the analog inputs look directly into the
input sampling capacitor, CSAMP. The dc input leakage current in
this unbuffered mode is 1 nA maximum. As a result, the analog
inputs see a dynamic load that is switched at the input sample
rate (see Figure 5). This sample rate depends on master clock
frequency and selected gain. CSAMP is charged to AIN(+) and
discharged to AIN() every input sample cycle. The effective
on-resistance of the switch, RSW, is typically 7 kΩ .
HIGH
IMPEDANCE
>1G
RSW (7k TYP)
CSAMP
(10pF)
VBIAS
SWITCHING FREQUENCY
DEPENDS ON
fCLKIN AND
SELECTED GAIN
AIN(+)
AIN(–)
08
51
9-
00
5
Figure 5. Unbuffered Analog Input Structure
CSAMP must be charged through RSW and through any external
source impedances every input sample cycle. Therefore, in
unbuffered mode, source impedances mean a longer charge
time for CSAMP, and this may result in gain errors on the part.
Table 24 shows the allowable external resistance/capacitance
values, for unbuffered mode, such that no gain error to the
16-bit level is introduced on the part. Note that these capacitances
are total capacitances on the analog input, external capacitance
plus 10 pF capacitance from the pins and lead frame of the device.
Table 24. External R, C Combination for No 16-Bit Gain
Error (Unbuffered Mode Only)
External Capacitance (pF)
Gain
10
50
100
500
1000
5000
1
152 kΩ
53.9 kΩ
31.4 kΩ
8.4 kΩ
4.76 kΩ
1.36 kΩ
2
75.1 kΩ
26.6 kΩ
15.4 kΩ
4.14 kΩ
2.36 kΩ
670 Ω
32
16.7 kΩ
5.95 kΩ
3.46 kΩ
924 Ω
526 Ω
150 Ω
128
16.7 kΩ
5.95 kΩ
3.46 kΩ
924 Ω
526 Ω
150 Ω
In buffered mode, the analog inputs look into the high impedance
inputs stage of the on-chip buffer amplifier. CSAMP is charged via
this buffer amplifier such that source impedances do not affect
the charging of CSAMP. This buffer amplifier has an offset leakage
current of 1 nA. In this buffered mode, large source impedances
result in a small dc offset voltage developed across the source
impedance but not in a gain error.
Input Sample Rate
The modulator sample frequency for the AD7715 remains at
fCLK IN/128 (19.2 kHz @ fCLK IN = 2.4576 MHz) regardless of the
selected gain. However, gains greater than 1 are achieved by a
combination of multiple input samples per modulator cycle and
a scaling of the ratio of reference capacitor to input capacitor. As
a result of the multiple sampling, the input sample rate of the device
varies with the selected gain (see Table 25). In buffered mode, the
input is buffered before the input sampling capacitor. In unbuffered
mode, where the analog input looks directly into the sampling
capacitor, the effective input impedance is 1/CSAMP × fS where
CSAMP is the input sampling capacitance and fS is the input
sample rate.
Table 25. Input Sampling Frequency vs. Gain
Gain
Input Sampling Frequency (fS)
1
fCLK IN/64 (38.4 kHz @ fCLK IN = 2.4576 MHz)
2
2 × fCLK IN/64 (76.8 kHz @ fCLK IN = 2.4576 MHz)
32
8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz)
128
8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz)
Bipolar/Unipolar Inputs
The analog input on the AD7715 can accept either unipolar or
bipolar input voltage ranges. Bipolar input ranges do not imply
that the part can handle negative voltages on its analog input
since the analog input cannot go more negative than 30 mV to
ensure correct operation of the part. The input channel is fully
differential. As a result, the voltage to which the unipolar and
bipolar signals on the AIN(+) input are referenced is the voltage
on the respective AIN() input. For example, if AIN() is
2.5 V and the AD7715 is configured for unipolar operation with
a gain of 2 and a VREF of 2.5 V, the input voltage range on the
AIN(+) input is 2.5 V to 3.75 V. If AIN() is 2.5 V and the
AD7715 is configured for bipolar mode with a gain of 2 and a
VREF of 2.5 V, the analog input range on the AIN(+) input is
1.25 V to 3.75 V (that is, 2.5 V ± 1.25 V). If AIN() is at AGND,
the part cannot be configured for bipolar ranges in excess of
±30 mV.
Bipolar or unipolar options are chosen by programming the
B/U bit of the setup register. This programs the channel for
either unipolar or bipolar operation. Programming the channel
for either unipolar or bipolar operation does not change any
of the input signal conditioning; it simply changes the data
output coding and the points on the transfer function where
calibrations occur.
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