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参数资料
| 型号： | SOIC14EV |
| 厂商： | Microchip Technology |
| 文件页数： | 10/22页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL 14SOIC/TSSOP/DIP |
| 标准包装： | 1 |
| 主要目的： | 裸评估板 |
| 嵌入式： | 否 |
| 主要属性： | 14 引脚 SOIC/DIP/TSSOP 评估板 |
| 次要属性： | 用于 PICmicro? MCU、ADC、DAC 和其它设备 |
| 已供物品： | 5 裸板，CD |
| 产品目录页面： | 677 (CN2011-ZH PDF) |
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)
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| SOIC-14PIN | 制造商:IRF 制造商全称:International Rectifier 功能描述:14-Pin Surface Mount, Narrow Body |
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