current through R1. The feedback " />
參數(shù)資料
型號： AD7877ACPZ-500RL7
廠商： Analog Devices Inc
文件頁數(shù)： 21/45頁
文件大?。?/td> 0K
描述： IC ADC 12BIT TOUCHSCREEN 32LFCSP
標準包裝： 1
類型： 電阻
觸摸面板接口： 4 線
輸入數(shù)/鍵： 1 TSC
分辨率（位）： 12 b
評估套件： 可供
數(shù)據(jù)接口： 串行，SPI?
數(shù)據(jù)速率/采樣率 (SPS，BPS)： 125k
電壓基準： 外部，內部
電源電壓： 2.7 V ~ 5.25 V
電流 - 電源： 1µA
工作溫度： -40°C ~ 85°C
安裝類型： 表面貼裝
封裝/外殼： 32-VFQFN 裸露焊盤，CSP
供應商設備封裝： 32-LFCSP-VQ
包裝： 標準包裝
其它名稱： AD7877ACPZ-500RL7DKR
Data Sheet
AD7877
Rev. D | Page 27 of 44
When the DAC output voltage is zero, it sinks the maximum
current through R1. The feedback current and, therefore, VOUT
are at their maximum. As the DAC output voltage increases, the
sink current and, thus, the feedback current decrease, and VOUT
falls. If the DAC output exceeds VREF, it starts to source current,
and VOUT has to further decrease to compensate. When the
DAC output is at full scale, VOUT is at its minimum.
Note that the effect of the DAC on VOUT is opposite in voltage
mode to that in current mode. In current mode, increasing
DAC code increases the sink current, so VOUT increases with
increasing DAC code. In voltage mode, increasing DAC code
increases the DAC output voltage, reducing the sink current.
Calculate the resistor values as follows:
1. Decide on the feedback current as before.
2. Calculate the parallel combination of R1 and R3 when the
DAC output is zero
RP = VREF/IFB
3. Calculate R2 as before, but use RP and VOUT(MAX)
R2 = RP(VOUT(MAX) VREF)/VREF
4. Calculate the change in feedback current between
minimum and maximum output voltages as before using
I = VR2(MAX)/R2 VR2(MIN)/R2
This is equal to the change in current through R1 between
zero output and full scale, which is also given by
I = current at zero current at full scale
= V/R1 (VREF V)/R1
= V/R1
5. R1 = VFS/.
6. Calculate R3 from R1 and R using
R3 = (R1 × RP)/(R1 RP)
Example:
1. VCC = 5 V and VFS = VCC. VOUT(MIN) is 20 V and VOUT(MAX) is
25 V. VREF is 1.25 V. Allow 100 A around the feedback loop.
2. RP = 1.25 V/100 A = 12.5 k.
3. R2 = 12.5 k × (25 1.25 )/1.25 = 237 k.
Use the nearest preferred value of 240 k.
4. I = 25 V/240 k 20 V/240 k = 21 A.
5. R1 = 5 V/21 A = 238 k.
Use the nearest preferred value of 250 k.
6. R3 = (180 k × 12.5 k)/(180 k 12.5 k) = 13.4 k.
Use nearest preferred value of 13 k.
The actual adjustment range using these values is 21 V to 26 V.
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