The LM4562 Opamp.

Updated: 15 Mar 2007

CONTENTS.

• Introduction.
• The Specs.
• Fig a. Shunt feedback: THD for various loads.
• Fig b. Series feedback: THD for various loads.
• Fig c. Series feedback: THD versus Rsource.

INTRODUCTION.
This is a brand-new opamp, which has just become freely available at the beginning of 2007.
The LM4562 is a National Semiconductor product. It is a dual opamp- there is no single or quad version.

Input noise voltage is typically 2.7 nV/rootHz, which is substantially lower than the 4 nV/rootHz of the 5532. For suitable applications- with low source impedances- this translates into a noise advantage of 3 dB or more. It is not fussy about decoupling, and as with the 5532, 100nF across the supply rails usually ensures stability. Whether decoupling from rails to ground is required depends on the application.
Slewrate is typically +/-20 V/us, but the minimum is a bit lower at +/-15 V/us.

No details of the internal circuitry have been released so far, and quite probably never will be.

The first two THD plots below show the device working at a closed-loop gain of 2.2x in shunt feedback mode and 3.2x in series feedback mode; both configurations having a noise gain of 3.2x. It is obvious that a problem emerges in the series plot, where the THD is higher by something like three times at 7.7 Vrms and 10 kHz. This distortion increases with level, which immediately suggests common-mode distortion in the input stage.

SPECS.
Here are the vital statistics: All typical values, for +/-15V supply rails.

Supply voltage	+/-18V abs max
Output range	+/-14V typ (2K load)
CM range	+14.1/-13.9V
en	2.7 nV/rtHz typ 1 kHz
in	1.6 pA/rtHz typ 1 kHz
Ibias	10 nA typ
Slew rate:	20 V/us
Supply current	10 mA total (quiescent)
Unity gain stable	YES
Cost	3$US Mar 2007

Above: The LM4562 working in shunt feedback mode (to remove input CM distortion) at a gain of 2.2x with various loads. Test level is 7.9 Vrms output. (approx +20 dBu) +/-15V supply rails.

The THD plot is indistinguishable from the analyser output when it has no external load. (The output is loaded by the 22K feedback resistance) With a heavy 470R load there is a small amount of extra THD above 15 kHz.

.

Above: The LM4562 in series feedback mode, gain 3.2x. 600 Ohm load and no load. Test level 7.9 Vrms output. +/-15V supply rails.

Note much higher distortion at HF compared with shunt feedback test. The feedback resistors were 22K and 10K, so the inverting input is presented with a source resistance of 22K||10K = 6.87 K.
The effect of 600 Ohm loading is very slight.

Above: The LM4562 in series feedback mode, gain 3.2x, with varying extra source resistance feeding the the inverting input. 7.7 Vrms out, +/-15V supply rails.

Here the feedback resistances were reduced to 2K2 and 1K, so the minimum source resistance presented to the inverting input is 687 Ohms. Extra series resistances were then put in series with the inverting input, so source resistance could be altered without affecting gain or output loading. A small amount of extra resistance (2K2) actually makes the THD fall at first, presumably because distortion fom the analyser is being cancelled out. After that, things get rapidly worse...
Note that only the rising curves to the right represent actual distortion. The rise in the horizontal traces at the LF end is due to increased noise.

This is not a unique problem with the LM4562. Applying the same test to the 5532 gives almost identical results.

THE COMMON-MODE PROBLEM.
What can be done about this unwelcome common-mode distortion? Obviously choosing a circuit configuration, such as shunt feedback, that minimises or eliminates the CM signal is one approach. However, if this is not possible, a small but dependable improvement can be gained by running the opamp off the highest supply rails permitted.