Semiconductor companies don’t always highlight the inner details of certain products. One insidious issue is the presence of sneak diodes which is a nasty issue that has been discussed before regarding a D/A converter, but now let’s look at another example of getting into trouble.
The following sketch is a low frequency clock oscillator that uses an op-amp. Typically, I would choose a type TL082 for this purpose. Also, just as a side note, this kind of oscillator was a key part of many of the high voltage power supplies made by Bertan High Voltage when I was employed there.
Figure 1 Low frequency clock oscillator that depends on the ability of U1 to accept large differential voltages between its input pins.
This circuit depends on the ability of U1 to accept large differential voltages between its input pins. Nominally, the junction of R1, R2 and R3 will step between Vcc/3 and 2*Vcc/3 and the top of C1 will swing back and forth between those two voltage levels along a time constant set by R4*C1.
The TL082 (Figure 2) is well suited to this purpose because there is nothing connected between its two input pins to restrict the differential input voltage from reaching the limits that I’ve just described.
Figure 2 The TL082 op-amp that is well-suited for the low frequency clock oscillator described in Figure 1.
However, not every op-amp has this property. As an example, please consider the Analog Devices OP184/284/84 op-amps in Figure 3.
Figure 3 The OP184/284/484 op-amps where the maximum possible differential voltage is limited by QL1 and QL2, these two act as paralleled back-to-back diodes.
The maximum possible differential voltage is limited by QL1 and QL2 which act as paralleled, back-to-back diodes. Those diodes limit the maximum differential input voltage. In linear service, these diodes being there probably wouldn’t matter, but the oscillator of Figure 1 is not a linear circuit.
Two SPICE simulations demonstrate the impact this diode difference has on the oscillator (Figure 4).
Figure 4 A comparison of the TL082 op-amp (U1) and the OP184/284/484 op-amps, showing the effect the diodes have on the oscillator.
The oscillation frequency with the two diodes present is almost an octave higher with those diodes than it would be without them.
The TL082 is by no means the only op-amp suitable for service in this kind of oscillator circuit, but not every op-amp is suitable for it. Non-linear circuits other than this kind of oscillator might be affected in some way as well.
Just be certain of your particular case(s).
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
Related Content
- Configuration control and second sourcing
- Choosing the correct ADC analog input type to reduce risk of redesign
- Designing with a DAC: Settle for the best
- Selecting op amps
- A designer’s guide to op-amp gain error
The post Sneak diodes and their impact on your designs appeared first on EDN.