Sometimes, it's necessary to make microvolt measurements in the presence of volts. For example, the need arose to measure the thermoelectric potentials in an RTD circuit in which the 10 V applied to self-heat the RTD made measuring such tiny voltages difficult. This circuit applies heating power for about 90% of the time and measures the RTD thermoelectric potential during the approximately 10% of off-time (Fig. 1).
Two switches are required. A power switch applies power but must turn off to a few nanoamperes during the power-off time. A measuring switch passes the RTD's tiny thermoelectric potential during power-off time, but it must reject the nearly 10 V present during power-on time. The applicable section of the circuit shows these switches (see the figure).
Because one ought not to rely on a warm transistor to pass no more than a few nanoamperes, Q4 and Q5 with R20 form a series-shunt power switch. Because Q4's off-current is very small, it only drops a tiny voltage across R20. Q5 then passes very little current, due to the fact that it has little voltage across it. D8 is chosen to have low leakage to avoid unwanted leakage current in the power-switch output. Baker clamps are applied to both Q4 and Q5 to ensure prompt switching. J1 and J5 form a series-shunt measuring switch.
Figure 2 shows the RTD heating waveform, the thermoelectric output waveform, and the waveforms needed to drive the switches. The waveforms are carefully sequenced to keep the thermoelectric output pure.
As the measuring switch output is already chopped, an ac amplifier and synchronous rectifier were used to condition its output. However, a precision amplifier followed by a sample-and-hold circuit would also be suitable.
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