CONTENTS.
- Introduction.
- The specs.
- Fig 1 Series feedback: THD versus level, no loading.
- Fig 2 Shunt feedback: THD versus loading at 3.0 Vrms o/p.
INTRODUCTION.
This opamp was recommended to me as a possible substitute for either the 5534 or the OPA2134. It was a big disappointment. Distortion is gratifyingly low at low frequencies but shoots up above 4 kHz. The rate of increase is faster than 18 dB/octave and the fastest I have ever seen on an opamp. (Slew-rate grief excepted) Possibly this points to something special in the way of internal compensation.
The claimed DC precision is most impressive (I didn't check it) but the excessive HF distortion rule this out for audio applications, except... it could be useful for DC servos, where DC accuracy is needed but no audio worth mentioning goes through the servo path.
An interesting point is that the data sheet remains silent on what technology is used. However, close scrutiny of the text unearthed the phrase "input stage base current" so it must be bipolar. The very low bias current may be the result of an exceptionally effective bias-current-cancellation system. Voltage noise density looks rather poor, being twice as as bad as the much, much older NE5534.
The two THD plots show the device working at a gain of 3.2x in both shunt and series feedback modes.
There appears to be no common-Mode (CM) distortion in this IC. Bipolar inputs always seem to be free of it.
SPECS.
Here are the vital statistics: all typical values, for +/-15V supply rails.
| Supply voltage | +/-18V abs max
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| Output range | +/-17V typ (2K load)
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| CM range | +/-13V
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| en | 8 nV/rtHz typ 1 kHz
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| in | 3 fA/rtHz typ
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| Ibias | 5 pA typ
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| Slew rate: | 20 V/us
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| Supply current | 8 mA total
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| Unity gain stable | YES
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| Cost | 114p RS Jan 2001
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Fig 1. Above: the OP2277 in series feedback mode, at various output levels, with no load. Note very fast rise in distortion at HF. The THD in the horizontal bits is below the noise until the plot turns upwards; it appears to be lower at 5 Vrms simply because the noise floor is relatively lower.
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Fig 2. Above: the OP2277 in shunt feedback mode, (to remove input CM distortion) and with nothing, 600 R and 150R loads on the output. As usual, more loading makes linearity worse. A relatively good performance into the very low load of 150R
3.0 Vrms out, Gain= 3.2x
The red lines show that THD is rising faster than 18 dB/octave.
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