Updated: 15 Jan 2001
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Updated: 15 Jan 2001
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INTRODUCTION.
The OP27 from Analog Devices is a bipolar-input single opamp primarily designed for low noise and DC precision. It was not intended for audio use, but in spite of this it is frequently recommended for such applications as RIAA and tape head preamps. This is unfortunate.
At first sight it appears that the OP27 is quieter than the 5534/5532; en is 3.2 nV/rtHz compared with 4 for the 5534. However, on practical measurement, the OP27 is in fact slightly noisier. This is believed to be because the OP27 has input bias-current circuitry that generates common-mode noise. When the impedances on the two inputs are very different- which is the case in RIAA preamps- the CM noise does not cancel, and this appears to be enough to degrade the overall noise performance significantly.
The maximum slew rate is not high compared with other opamps, being typically 2.8V/us. However, this is not the problem it may appear. This slew rate would allow a maximum amplitude at 20 kHz of 16 Vrms, if the supply rails permitted it.
SPECS.
Here are the vital statistics:
All typical values, for +/-15V supply rails.
| Supply voltage | +/-22V abs max |
| Output range | +/-13.8V typ (2K load) |
| CM range | +/-12.3V |
| Ibias | 10nA typ |
| en | 3.0 nV/rtHz typ 1 kHz |
| in | 0.4 pA/rtHz typ |
| Slew rate: | 2.8 V/us typ |
| Supply current | 3 mA |
| Unity gain stable | YES |
| Cost | 215p RR Jan 2001 |
DECOUPLING AND STABILITY.
No particular difficulties were encountered.
| THD plot not available at present. |
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