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Recycle precision potentiometers as useful voltage sources

An analog- or a mixed-signal lab cannot have too many voltage sources. A simple, reasonably high-precision voltage source can set bias points in an op-amp circuit, tweak the feedback node of a power supply through a large resistor, or run a quick linearity test on an ADC. Engineers often use a dcpower supply because it is the only thing they can find, and many labs lack a true voltage-calibration source. This Design Idea describes a circuit that recycles old precision potentiometers that have directreading scales into useful laboratory“volt boxes.”

Several types of potentiometers work in the circuit in Figure 1. Standard 10-turn potentiometers typically have 0.1% linearity and work well for general-purpose tweaking. However, a five-decade Kelvin-Varley divider with a total resistance of 100 k
or less achieves 10-ppm accuracy.

Having some indication from a voltage source that its output is correct proves useful. A digital panel meter is one way of achieving this goal. However, even a 0.1% potentiometer is more accurate than most of these meters. Consequently, to indicate that the output is correct, you need to know only whether the power is on, whether the supply voltage is high enough, and whether the output amplifier is working properly, not sourcing or sinking too much current, oroscillating.

A single red-green-blue LED provides all three indications. The green LED flashes at a low duty cycle when the power is on and stays lit continuously when the battery voltage is too low. The red LED illuminates when the output is out of regulation because IC4B is a low-dutycycle relaxation oscillator that pulses a green LED for 5 msec at approximately 0.5 Hz. The blue LED lights when the voltage source is sinking too much current. If the output isoscillating, the LED glows purple.

IC_4A compares the positive battery voltage to the precision 10V reference output and continuously turns on the green LED when the positive battery voltage drops below 11.5V: as this level is the dropout voltage of the reference, you know it’s time to change the batteries. The load on the positive supply is greater than that on the negative supply, so these cells wear out first. And, because only two cells constitute the negative supply, battery wastage is minimal. Alternatively, you can move the negative cells to the positive side to squeezethe last bit of juice from them.

The reference is IC₁, an LT1236-10 with an added trim circuit. The LT1236 is quiet and stable over time and temperature. Its output drives the top of the precision potentiometer or Kelvin-Varley divider. The output of the circuit is trimmed to 10V when the potentiometer or divider is at its maximum value. The two halves of an LT1881 amplifier, IC_2A and IC_2B, buffer the output of the potentiometer or divider. The combined bias current for both buffers is 400 pA maximum, which causes a change of approximately 10 µV in the output voltage of a 100-kΩ potentiometer when it is at midscale. Make sure to properly guard the noninverting inputs to prevent leakage. The 50-µV maximum offset and 130-dB CMRR (commonmode- rejection ratio) keep overall accuracy well within 10 ppm of a10V total span.

One-half of the LT1881 is the voltage output of the volt box. The other half is necessary to drive the two inputs to IC₅, an LT1017 dual comparator that has an input-bias current of 15 nA per comparator. Q₁ to Q₆ form a 100-µA current sink andsource referred to the negative supply and positive supply, respectively. Youadjust potentiometers R₁ and R₂ to setup a window around the output voltagethat is compared with the outputof IC_2A, which is a replica of the correctoutput voltage. If IC_2B is sourcingor sinking too much current, oneof the comparators will trip, turningon the respective LED. If the outputis oscillating, both LEDs will light.The window is adjustable from 0 toapproximately ±9.3 mV; ±1 mV is agood place to start.

Should you need more output current than the 5 mA that the LT1881 guarantees, you can switch in an LT1010 buffer to provide a “turboboost” feature, increasing the outputcurrent capability to ±150 mA and greatly enhancing the ability to drive capacitive loads. In normal operating conditions, you should disable this buffer because it draws 10 mA more from the supply. Switch S₃ allows reverse polarity, and, if you use a center-off switch, you can disconnect the output. S₁ is the power switch and can also select power from an external supply or batterypower when isolation is critical.