Semiconductor firms don’t all the time spotlight the inside particulars of sure merchandise. One insidious concern is the presence of sneak diodes which is a nasty concern that has been mentioned earlier than relating to a D/A converter, however now let’s have a look at one other instance of entering into bother.
The next sketch is a low frequency clock oscillator that makes use of an op-amp. Usually, I might select a kind TL082 for this objective. Additionally, simply as a aspect observe, this type of oscillator was a key a part of most of the excessive voltage energy provides made by Bertan Excessive Voltage after I was employed there.
Determine 1 Low frequency clock oscillator that is determined by the flexibility of U1 to just accept massive differential voltages between its enter pins.
This circuit is determined by the flexibility of U1 to just accept massive differential voltages between its enter pins. Nominally, the junction of R1, R2 and R3 will step between Vcc/3 and a couple of*Vcc/3 and the highest of C1 will swing forwards and backwards between these two voltage ranges alongside a time fixed set by R4*C1.
The TL082 (Determine 2) is nicely suited to this objective as a result of there may be nothing linked between its two enter pins to limit the differential enter voltage from reaching the boundaries that I’ve simply described.
Determine 2 The TL082 op-amp that’s well-suited for the low frequency clock oscillator described in Determine 1.
Nevertheless, not each op-amp has this property. For example, please take into account the Analog Units OP184/284/84 op-amps in Determine 3.
Determine 3 The OP184/284/484 op-amps the place the utmost doable differential voltage is restricted by QL1 and QL2, these two act as paralleled back-to-back diodes.
The utmost doable differential voltage is restricted by QL1 and QL2 which act as paralleled, back-to-back diodes. These diodes restrict the utmost differential enter voltage. In linear service, these diodes being there in all probability wouldn’t matter, however the oscillator of Determine 1 isn’t a linear circuit.
Two SPICE simulations display the influence this diode distinction has on the oscillator (Determine 4).
Determine 4 A comparability of the TL082 op-amp (U1) and the OP184/284/484 op-amps, displaying the impact the diodes have on the oscillator.
The oscillation frequency with the 2 diodes current is nearly an octave larger with these diodes than it could be with out them.
The TL082 is in no way the one op-amp appropriate for service in this type of oscillator circuit, however not each op-amp is appropriate for it. Non-linear circuits apart from this type of oscillator could be affected indirectly as nicely.
Simply be sure of your specific case(s).
John Dunn is an electronics advisor, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York College (MSEE).
Associated Content material
- Configuration management and second sourcing
- Selecting the right ADC analog enter kind to cut back danger of redesign
- Designing with a DAC: Accept one of the best
- Deciding on op amps
- A designer’s information to op-amp acquire error
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The put up Sneak diodes and their influence in your designs appeared first on EDN.
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