LMV751

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SNOS468E – AUGUST 1999 – REVISED MARCH 2013

Actual resistor noise measurements may have more noise than the calculated value. This additional noise

component is known as excess noise. Excess noise has a 1/f spectral response, and is proportional to the

voltage drop across the resistor. It is convenient to define a noise index when referring to excess noise in

resistors. The noise index is the RMS value in uV of noise in the resistor per volt of DC drop across the resistor

in a decade of frequency. Noise index expressed in dB is:

where

•

•

(2)

Excess noise in carbon composition resistors corresponds to a large noise index of +10 dB to -20 dB. Carbon

film resistors have a noise index of -10 dB to -25 dB. Metal film and wire wound resistors show the least amount

of excess noise, with a noise index figure of -15 dB to -40 dB.

Other noise sources:

As the op amp and resistor noise sources are decreased, other noise contributors will now be noticeable. Small

air currents across thermocouples will result in low frequency variations. Any two dissimilar metals, such as the

lead on the IC and the solder and copper foil of the pc board, will form a thermocouple. The source itself may

also generate noise. An example would be a resistive bridge. All resistive sources generate thermal noise based

on the same equation listed above under "resistor types". (2)

Putting it all together

To a first approximation, the total input referred noise of an op amp is:

n

req

2

where

•

Req is the equivalent source resistance at the inputs

(3)

At low impedances, voltage noise dominates. At high impedances, current noise dominates. With a typical noise

current on most CMOS input op amps of 0.01 pA/

√Hz, the current noise contribution will be smaller than the

voltage noise for Req less than one megohm.

Other Considerations

Comparator operation

Occasionally operational amplifiers are used as comparators. This is not optimum for the LMV751 for several

reasons. First, the LMV751 is compensated for unity gain stability, so the speed will be less than could be

obtained on the same process with a circuit specifically designed for comparator operation. Second, op amp

output stages are designed to be linear, and will not necessarily meet the logic levels required under all

conditions. Lastly, the LMV751 has the newer PNP-NPN common emitter output stage, characteristic of many

rail-to-rail output op amps. This means that when used in open loop applications, such as comparators, with very

light loads, the output PNP will saturate, with the output current being diverted into the previous stage. As a

result, the supply current will increase to the 20-30 mA. range. When used as a comparator, a resistive load

between 2k

Ω and 10kΩ should be used with a small amount of hysteresis to alleviate this problem. When used

as an op amp, the closed loop gain will drive the inverting input to within a few millivolts of the non-inverting

input. This will automatically reduce the output drive as the output settles to the correct value; thus it is only when

used as a comparator that the current will increase to the tens of milliampere range.

Rail-to-Rail

Because of the output stage discussed above, the LMV751 will swing “rail-to-rail” on the output. This normally

means within a few hundred millivolts of each rail with a reasonable load. Referring to the Electrical

Characteristics table for 2.7V to 5.0V, it can be seen that this is true for resistive loads of 2k

Ω and 10kΩ. The

input stage consists of cascoded P-channel MOSFETS, so the input common mode range includes ground, but

typically requires 1.2V to 1.3V headroom from the positive rail. This is better than the industry standard LM324

and LM358 that have PNP input stages, and the LMV751 has the advantage of much lower input bias currents.

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