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18
Digital Potentiometer Design Guide
The following circuit shows a Wheatstone Bridge with current
limiting. In a Wheatstone Bridge, there are four resistive
elements. In this example, two are fixed value (R1 and R2),
there is a resistive sensor (RSENSOR) and then there is the
digital potentiometer in rheostat configuration to calibrate the
circuit due to variations of the resistive sensor. This sensor
could be for temperature or weight measurement.
At a default condition the sensor should be a given value, but
this value will change from device to device. To compensate
for the resistive changes in the R1 plus RSENSOR leg of the
bridge, the Rheo2 would be modified for the R2 plus Rheo2
leg of the bridge. This would be done so that the voltages of
VBRG1 and VBRG2 are at their desired levels. Many times this
is VBRG1 = VBRG2.
Now as the conditions on the sensor change, the resistance
of the sensor will change, causing the VBRG2 voltage to
change. The delta voltage between VBRG1 and VBRG2 can then
be used to determine the state of the system (temperature,
weight, etc.).
Rheo 1 is used in a rheostat mode to limit the current or trim
the current through the Wheatstone Bridge.
Implementing a More Precise Rheostat
The RAB value of a typical digital potentiometer can vary as
much as ±20%, so a device with a 10 kΩ RAB value could
have an RAB value as small as 8 kΩ, as large as 12 kΩ. In
a system, this variation for the rheostat value may not be
desirable. This variation can be calibrated out to make a
precise rheostat, at a cost of the resolution of the device.
If we design the application circuit where this rheostat
only operates from 0Ω to 8 kΩ, all digital potentiometer
devices (over process) will meet this requirement. Now with
calibration, we will need to ensure that the wiper value is
limited to a value where the rheostat value is the closest
resistance value to the desired rheostat target value of 8
kΩ. The worst case (lowest) wiper value occurs when the RAB
value is 12 kΩ. In this case, a wiper value of 171. results
in a resistance of 8016Ω. This results in a resolution of
approximately 7.4 bits, or 0.58%.
In potentiometer mode, the process variation of the RAB value
may not be an application issue since the device is operating
as a voltage divider.
Logarithmic Steps
Logarithmic steps are desirable since the human ear hears
in a logarithmic manner. The use of a linear potentiometer
can approximate a log potentiometer, but with fewer steps.
An 8-bit potentiometer can achieve fourteen 3 dB log steps
plus a 100% (0 dB) and a mute setting.
The figure below shows a block diagram of one of the
MCP44X1 resistor networks being used to attenuate an
input signal. In this case, the attenuation will be ground
referenced. Terminal B can be connected to a common mode
voltage, but the voltages on the A, B and Wiper terminals
must not exceed the MCP44X1 device’s VDD/VSS voltage
limits.
The following equation shows how to calculate voltage dB
gain ratios for the digital potentiometer.
More detail on this can be found in Section 8.5 of the
MCP444X/446X Data Sheet (DS22265).
Wheatstone Bridge Trimming
Digital Potentiometer Solutions
Signal Attenuation Block Diagram - Ground Referenced
dB Calculations (Voltage)
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| MCP4451-103E/ML | IC POT 10K QUAD 8BIT 20QFN |
| VE-2NY-MX-F1 | CONVERTER MOD DC/DC 3.3V 49.5W |
| MCP4451-104E/ST | IC POT 100K QUAD 8BIT 20TSSOP |
| VE-23P-MY-F3 | CONVERTER MOD DC/DC 13.8V 50W |
| VE-2NX-MX-B1 | CONVERTER MOD DC/DC 5.2V 75W |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| MCP4451T-502E/ST | 制造商:Microchip Technology Inc 功能描述:5K, I2C, QUAD CHANNEL POT, 8-BIT, VOLATILE MEMORY, TSSOP - Tape and Reel |
| MCP4452-103E/ST | 功能描述:數(shù)字電位計(jì) IC 10k I2Cquad Ch Rheo 8bit volatile memory RoHS:否 制造商:Maxim Integrated 電阻:200 Ohms 溫度系數(shù):35 PPM / C 容差:25 % POT 數(shù)量:Dual 每 POT 分接頭:256 弧刷存儲(chǔ)器:Volatile 緩沖刷: 數(shù)字接口:Serial (3-Wire, SPI) 描述/功能:Dual Volatile Low Voltage Linear Taper Digital Potentiometer 工作電源電壓:1.7 V to 5.5 V 電源電流:27 uA 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TQFN-16 封裝:Reel |
| MCP4452-104E/ST | 功能描述:數(shù)字電位計(jì) IC 100k I2Cquad Ch Rheo 8bit volatile memory RoHS:否 制造商:Maxim Integrated 電阻:200 Ohms 溫度系數(shù):35 PPM / C 容差:25 % POT 數(shù)量:Dual 每 POT 分接頭:256 弧刷存儲(chǔ)器:Volatile 緩沖刷: 數(shù)字接口:Serial (3-Wire, SPI) 描述/功能:Dual Volatile Low Voltage Linear Taper Digital Potentiometer 工作電源電壓:1.7 V to 5.5 V 電源電流:27 uA 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TQFN-16 封裝:Reel |
| MCP4452-502E/ST | 功能描述:數(shù)字電位計(jì) IC 5k I2Cquad Ch Rheo 8bit volatile memory RoHS:否 制造商:Maxim Integrated 電阻:200 Ohms 溫度系數(shù):35 PPM / C 容差:25 % POT 數(shù)量:Dual 每 POT 分接頭:256 弧刷存儲(chǔ)器:Volatile 緩沖刷: 數(shù)字接口:Serial (3-Wire, SPI) 描述/功能:Dual Volatile Low Voltage Linear Taper Digital Potentiometer 工作電源電壓:1.7 V to 5.5 V 電源電流:27 uA 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TQFN-16 封裝:Reel |
| MCP4452-503E/ST | 功能描述:數(shù)字電位計(jì) IC 50k I2Cquad Ch Rheo 8bit volatile memory RoHS:否 制造商:Maxim Integrated 電阻:200 Ohms 溫度系數(shù):35 PPM / C 容差:25 % POT 數(shù)量:Dual 每 POT 分接頭:256 弧刷存儲(chǔ)器:Volatile 緩沖刷: 數(shù)字接口:Serial (3-Wire, SPI) 描述/功能:Dual Volatile Low Voltage Linear Taper Digital Potentiometer 工作電源電壓:1.7 V to 5.5 V 電源電流:27 uA 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TQFN-16 封裝:Reel |
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