Photocell makes true-zero output of the op-amp

While choosing an op-amp buffer for a new high-resolution single-supply DAC, a source of negative supply was considered because the buffer op-amp had to provide true zero voltage on its output.

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For instance, a typical rail-to-rail output op-amp can’t provide true zero voltage, it can guarantee at least several mV on its output, while a high-resolution DAC can have resolution in the of tens microvolts. The application required a true zero output, hence the problem.

For sure, some negative supply source was needed to increase the “headroom” around zero. (I use the term “headroom” because we are dealing not with an upper, positive supply, but with lower one. A better word would be “footroom”.)

There was an intention to use the Cuk configuration circuit again, like the old circuit in EDN, but with an output voltage of about -1 V only and a low—less than 2 mA—output current.

While exploring alternatives, the idea arose to use a photocell instead of any ordinary voltage converter. It resulted in the circuit in Figure 1.

Figure 1 Using photocells instead of a voltage converter to help provide a true zero volage on the output of an op-amp buffer for a high-resolution single-supply DAC.

The solution has comparable dimensions with the circuit based on the Cuk configuration, albeit lower efficiency. But since the superfluous power doesn’t excess 0.1 W, this may be of no importance.

Such a solution has important advantages:

It’s far simpler.
It produces very low electrical noise—a fact of great significance when you are dealing with low analog signals. (In this circuit the output noise was less than 1 mV even without output capacitor C1.)
Any over-voltages on its output are excluded (while the Cuk converter can produce such over-voltage if any problems with feedback occurs).
The perfect level of isolation should also be noted, albeit it is not important in our case.

Since the external outlines of the gadget are determined by the photocell, the tiny photocell AM-1417 (of Toshiba) was used. Its dimensions are only 34 x 14 x 2 mm, and 4 sections it has—hence 4 LEDs, one for each section—produce about 3 V without any load.

The 4 LEDs are quite ordinary ones of bright red family (L-513HURC, 1800 mcd in 15° angle) because Si photocell has its maximal efficiency in this area of spectrum.

Reds are also preferable for +5 V supply since their low forward voltage allows to double the efficiency very simply, by stacking them in pairs with the same current through both.

The circuit produces 490…520 mV on 2k load @ 20 mA current through the LED. This is more than enough for several micropower op-amps such as the AD8603/AD8607.

The output voltage of the photocell can be varied by changing the current through LEDs.

The photocell is a current—not voltage—source, so the capacitor C1 is required to reduce the output impedance of the circuit. Diode D1 enables a path for sinking current and protects this electrolytic capacitor if the negative voltage for some reason disappears.

As I mentioned, the output power is quite enough for a precision micropower op-amp, such as the AD8603, for example. If you need more power, you can use higher current through LEDs, more efficient pair LED/photocell, or simply connect more such circuits in parallel.

—Peter Demchenko studied math at the University of Vilnius and has worked in software development.

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