Datasheet

LMV751
www.ti.com 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:
NI = 20 log ((E
EX
/V
DC
) x 10
6
) db
where
• E
EX
= resistor excess noise in uV per frequency decade
• V
DC
= DC voltage drop across the resistor (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:
E
t
2
= e
n
2
+ e
req
2
+ (i
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 2kΩ 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 2kΩ 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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