电路实现基于PC的电压测量-Circuit Enables
abstract: in its simplest form, the pc voltmeter consists of a 4-channel, 10-bit a/d converter (ic2) with a few passive components, connected to the pc printer port via a 25-pin male d-type connector.
in its simplest form, the pc voltmeter of figure 1 consists of section a alone: a 4-channel, 10-bit a/d converter (ic2) with a few passive components, connected to the pc printer port via a 25-pin male d-type connector. power is derived from pins 2 and 3, and the circuit draws about 1ma.
figure 1. this circuit and the associated q-basic listing enable a pc to monitor and measure the voltages on four input channels.
ic2's wide power-supply range (2.7v to 5.25v) allows operation from a variety of pc printer ports. the voltage at pins 2 and 3 is about 5v for a lot of desktop machines, but for portables it is more likely to be around 3v. ic2 handles them all. r10-r11 and c7-c8 filter the pc noise from this supply line. c6 and c9 decouple the 2.5v reference internal to ic2.
to maintain 10-bit accuracy, the source impedances driving the adc inputs (pins 2-5) must be less than 3kω. if not, you can buffer these inputs by adding the b section to the circuit. this quad op amp (configured as four noninverting buffers) has rail-to-rail inputs and outputs, operates with supply voltages down to 2.7v, and draws about 25µa per amplifier.
to provide high-voltage protection, the op amps' very low bias current (1pa typical) allows 180kω resistors in series with each input without introducing excessive error (r1-r4 in section c). for additional (anti-aliasing) filtering you can add capacitors c1-c4, and to increase the full-scale range from 2.5v to 25v you can add r5-r8.
other variations: you can substitute a 12-bit max1246 for ic2 in place of the 10-bit max1248, but for 12-bit accuracy you must restrict the supply voltage to the range 2.7v to 3.6v. alternatively, you can substitute a 12-bit max1247 and add an external reference such as the max872, which gives a 2.5v output for input voltages down to 2.7v.
two q-basic listings are available, each written for the 12-bit circuit options. listing 1 scans the four channels and displays each input voltage on the screen. listing 2 displays the channel voltages, plots them vs. time on the screen, and lets you save the data to disk.
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in its simplest form, the pc voltmeter of figure 1 consists of section a alone: a 4-channel, 10-bit a/d converter (ic2) with a few passive components, connected to the pc printer port via a 25-pin male d-type connector. power is derived from pins 2 and 3, and the circuit draws about 1ma.
figure 1. this circuit and the associated q-basic listing enable a pc to monitor and measure the voltages on four input channels.
ic2's wide power-supply range (2.7v to 5.25v) allows operation from a variety of pc printer ports. the voltage at pins 2 and 3 is about 5v for a lot of desktop machines, but for portables it is more likely to be around 3v. ic2 handles them all. r10-r11 and c7-c8 filter the pc noise from this supply line. c6 and c9 decouple the 2.5v reference internal to ic2.
to maintain 10-bit accuracy, the source impedances driving the adc inputs (pins 2-5) must be less than 3kω. if not, you can buffer these inputs by adding the b section to the circuit. this quad op amp (configured as four noninverting buffers) has rail-to-rail inputs and outputs, operates with supply voltages down to 2.7v, and draws about 25µa per amplifier.
to provide high-voltage protection, the op amps' very low bias current (1pa typical) allows 180kω resistors in series with each input without introducing excessive error (r1-r4 in section c). for additional (anti-aliasing) filtering you can add capacitors c1-c4, and to increase the full-scale range from 2.5v to 25v you can add r5-r8.
other variations: you can substitute a 12-bit max1246 for ic2 in place of the 10-bit max1248, but for 12-bit accuracy you must restrict the supply voltage to the range 2.7v to 3.6v. alternatively, you can substitute a 12-bit max1247 and add an external reference such as the max872, which gives a 2.5v output for input voltages down to 2.7v.
two q-basic listings are available, each written for the 12-bit circuit options. listing 1 scans the four channels and displays each input voltage on the screen. listing 2 displays the channel voltages, plots them vs. time on the screen, and lets you save the data to disk.
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