Texas Instruments | Microvolt Comparator | Application notes | Texas Instruments Microvolt Comparator Application notes

Texas Instruments Microvolt Comparator Application notes
LM111,LM321
Microvolt Comparator
Literature Number: SNOA858
National Semiconductor
Linear Brief 32
June 1976
Introduction
100. The 100k output impedance of the LM121A is shunted
by CS to filter out pickup and internally generated noise. No
feedback to the inputs of the pre-amp is employed to avoid
degrading common-mode rejection of the system.
Comparison of dc signal levels within microvolts of each
other can be made by using an LM121A pre-amp and an
LM111 comparator IC. Implementing this with two separate
IC’s decreases noise, eliminates troublesome thermal effects, and achieves a maximum offset drift of 0.22 µV/˚C
(Figure 1).
Designing a practical comparator with a voltage gain of 10
million involves protecting the input stage from temperature
changes or gradients, and avoiding problems of including
the noise filter within the positive feedback loop. The circuit
as shown has a 5 µV hysteresis which can be trimmed to
1 µV under certain conditions. Further, delays decrease with
increasing overdrive (see chart) due to elimination of input
stage thermal effects, saturating stages, and dielectric soak
or polarization effects on signal filter capacitors Table 1.
Designing with a Pre-Amp
With the bias network shown, the LM121A input stage has
an open-loop temperature stable voltage gain of close to
The separate pre-amp with a gain of 100 provides two major
advantages over single comparator designs. First, VOS and
other small errors attributed to the LM111 are reduced by the
100 gain factor. More important, temperature gradient
changes which occur within the LM111 when switching any
output load, are completely isolated by the separate packages and do not affect the pre-amp. If the entire microvolt
comparator were on a single silicon chip, a temperature
variation of as little as 1/1000˚C across the input stage could
have a significant effect.
This effect is a major reason for designing circuits sensitive
and stable to microvolt dc signals with a separate
pre-amplifier. Further, the special 4-transistor input stage,
when adjusted to zero offset with the “balance” control between pins 5 and 6, automatically reduces VOS change with
temperature to almost zero.
Microvolt Comparator
Microvolt Comparator
TABLE 1. Typical Overdrive Delays
Hyst.
RH
RS
CS
Set
5 µV
75 kΩ
10 kΩ
6800 pF
Delays with Various Overdrives
25%
100%
1000%
100 mV
2 ms
1.8 ms
600 µs
560 µs
Max.
00873301
FIGURE 1. Schematic Diagram
Filtering
© 2002 National Semiconductor Corporation
AN008733
www.national.com
LB-32
The pre-amp/comparator system generates a continuous
stream of very fast pulses if assembled without a filter, even
with positive feedback for hysteresis. This is caused by both
stray output-input feedback, and noise. The noise is both
thermal and pickup from the environment, including power
switching transients and fluorescent light hash. To cure this,
shunt filter capacitor CS is used.
Placing this capacitor outside the positive feedback loop has
two advantages. It eliminates a tendency for the comparator
Microvolt Comparator
Filtering
(Continued)
to oscillate during slow transitions. Also, response time to
small signals is halved since the positive hysteresis feedback signal is not stored on the filter capacitor.
A higher frequency filter (Cf) is needed to provide a low
impedance shunt to any high frequency noise and stray
feedback that may be picked up between LM111 terminals 5
and 6. These two terminals have almost the same voltage
sensitivity as the normal input terminals. The positive feedback to terminal 5, as described below, is only delayed
slightly by this filter.
Feedback
The positive feedback provided by the 5.1k/33Ω voltage
divider with RH is needed to insure clean, rapid changes of
state. It is applied to one of the “balance” terminals (pin 5) of
the LM111 to simplify the circuit over a balanced feedback
network, and to minimize signal stored on CS as previously
described. The current fed back to terminal 5 is single ended
with respect to the balance adjust network between these
terminals, and hence injects a dc offset of the desired polarity and amplitude for a few microvolts of latching.
Performance
A tabulation is shown for one of the many possible combinations of input circuits, filters, etc. For large amplitude
signals, CS can be decreased and hysteresis increased for
greater speed. Conversely, to obtain hysteresis as low as
1 µV, trim RH (to about 300k) use a CS of 0.01 µF to 0.1 µF
and have a low impedance source of signals.
For reduced ambient range and drift specifications, an
LM321 can be paired with the LM311 for a cost saving while
maintaining the same comparison sensitivity.
Design Tips for Microvolt Signals
Even with high performance devices such as the LM121,
microvolts of error can occur from thermocouple effects,
common-mode signals, “microphonics,” or unbalances in the
input or nulling circuits. As pointed out in Application Note
AN-79, Kovar lead to copper circuit board thermocouple
effects can cause a 3.5 µV offset voltage for only 0.1˚C
difference across the input leads. A compact layout of input
connections and shielding from air currents will minimize this
problem.
Although the LM121A has excellent common-mode rejection
( > 120 dB), a 1V change in common-mode voltage can
induce up to 1 µV of error voltage. For this reason
common-mode voltage changes should be kept to a minimum. Also, common-mode voltages allow mechanical vibrations in the probe cable to induce “microphonic” noise signals. Short, stiff, low capacitance and symmetrical input
shielded wires are recommended.
If it is possible to have a signal source balanced with regard
to ground, it will help decrease errors due to bias currents,
and noise due to common-mode and microphonic effects.
Matched, low temperature coefficient parts should be used in
the balance network, and care should be exercised in shielding input circuits and eliminating ground-loops.
Applications
The microvolt comparator is particularly well suited to controllers or test equipment having thermocouples or strain
gauges as inputs. This includes wind speed indicators, RMS
to dc converters, vacuum gauges, gas analysis equipment,
conductivity gauges, and hot wire controls. The strain
gauges can be used in materials testing, electronic weighing, pressure transducers, and load limiting sensors for
cranes, hoists, and rolling mills.
As a temperature controller, 1⁄8 degree or less on-off differential can be obtained using thermocouple types E, J, T or K.
Other microvolt signals used for control may come from Hall
effect sensors, Bolometers, slide-wires, and heat-flow thermopiles. A microvolt comparator will be useful in “Go/No-Go”
testing of low resistances such as switch and relay contacts,
RTDs,
coil
and
fuse
resistances,
and
pressure-sensitive-plastic conductors.
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LB-32
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