LMV751 Datasheet, PDF (7/8 Pages) National Semiconductor (TI) – Low Noise, Low Vos, Single Op Amp

Application Hints (Continued)
1.6 Putting it all together
To a first approximation, the total input referred noise of an
op amp is:
E_t2 = e_n2 + e_req2 + (i_n*Req)²
where Req is the equivalent source resistance at the inputs.
At low impedances, voltage noise dominates. At high imped-
ances, current noise dominates. With a typical noise current
on most CMOS input op amps of 0.01 pA/rt-Hz, the current
noise contribution will be smaller than the voltage noise for
Req less than one megohm.
2.0 Other Considerations
2.1 Comparator operation
Occasionally operational amplifiers are used as compara-
tors. This is not optimum for the LMV751 for several rea-
sons. First, the LMV751 is compensated for unity gain stabil-
ity, so the speed will be less than could be obtained on the
same process with a circuit specifically designed for com-
parator operation. Second, op amp output stages are de-
signed 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, charac-
teristic 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 hyster-
esis 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 re-
duce the output drive as the output settles to the correct
value; thus it is only when used as a comparator that the cur-
rent will increase to the tens of milliampere range.
2.2 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 in-
cludes ground, but typically requires 1.2V to 1.3V headroom
from the positive rail. This is better than the industry stan-
dard LM324 and LM358 that have PNP input stages, and the
LMV751 has the advantage of much lower input bias cur-
rents.
2.3 Loading
The LMV751 is a low noise, high speed op amp with excel-
lent phase margin and stablility. Capacitive loads up to 1000
pF can be handled, but larger capacitive loads should be iso-
lated from the output. The most straightforward way to do
this is to put a resistor in series with the output. This resistor
will also prevent excess power dissapation if the output is ac-
cidentally shorted.
2.4 General Circuits
With the low noise and low input bias current, the LMV751
would be useful in active filters, integrators, current to volt-
age converters, low frequency sine wave generators, and in-
strumentation amplifiers. (3)
Note: 1. Sherwin, Jim “Noise Specs Confusing?” AN-104, National Semicon-
ductor.
2. Christensen, John, “Noise-figure curve ease the selection of
low-noise op amps”, EDN, pp 81-84, Aug. 4, 1994
3. “Op Amp Circuit Collection”, AN-31, National Semiconductor.
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