LM363 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – LM363 Precision Instrumentation Amplifier

Application Hints (Continued)
Because the LM363’s output voltage is approximately one
diode drop below the voltage at pin 15 (pin 8 for the 8-pin
device) this point may be used to limit output swing as seen
in Figure 7a Current available from this pin is only 50 mA so
that zeners must have a sharp breakdown to clamp accu-
rately Alternatively a diode tied to a voltage source could
be used as in Figure 7b
50 pF to ground at both shield driver outputs Do not use
only one shield driver for a single-ended signal as oscilla-
tions can result shield driver to input capacitance must be
roughly balanced (g30%) To further reduce noise pickup
the shielded signal lines may be enclosed together in a
grounded shield If a large amount of RF noise is the prob-
lem the only sure cure is a filter capacitor at both inputs
otherwise the RFI may be internally rectified producing an
offset
DC loading on the shield drivers should be minimized The
drivers can only source approximately 40 mA above this
value the input stage bias voltages change degrading V_OS
and CMRR While the shield drivers can sink several mA
V_OS may degrade severely at loads above 100 mA (see
Shield Driver Loading Error curve in Typical Performance
Characteristics) Because the shield drivers are one diode
drop above the input levels unbalanced leakage paths from
shield to input can produce an input offset at high source
impedances Buffering with emitter-followers (Figure 8b ) re-
duces this leakage current by reducing the voltage differen-
tial and eliminates any loading on the amplifier
FIGURE 7 Output Clamp
TL H 5609 – 15
SHIELD DRIVERS
When differential signals are sent through long cables three
problems occur First noise both common-mode and differ-
ential is picked up Second signal bandwidth is reduced by
the RC low-pass filter formed by the source impedance and
the cable capacitance Finally when these RC time con-
stants are not identical (unbalanced source impedance
and or unbalanced capacitance) AC common-mode rejec-
tion is degraded amplifying both induced noise and
‘‘ground’’ noise Either filtering at the amplifier inputs or
slowing down the amplifier by overcompensating will indeed
reduce the noise but the price is slower response The
LM363D’s dual shield drivers can actually increase band-
width while reducing noise
The way this is done is by bootstrapping out shield capaci-
tance The shield drivers follow the input signal Since both
sides of the shield capacitance swing the same amount it is
effectively out of the circuit at frequencies of interest
Hence the input signal is not rolled off and AC CMRR is not
degraded (Figure 8 ) The LM363D’s shield drivers can han-
dle capacitances (shield to center conductor) as high as
1000 pF with source resistances up to 100 kX
For best results identical shielded cables should be used
for both signal inputs although small mismatches in shield
driver to ground capacitance (^s500 pF) do not cause prob-
lems At certain low values of cable capacitance (50 pF –
200 pF) high frequency oscillations can occur at high
source resistance (^t 10 kX) This is alleviated by adding
TL H 5609 – 16
FIGURE 8 Driving Shielded Cables
MISCELLANEOUS TRIMMING
The V_OS adjust and shield driver pins available on the 16-
pin package may be used to trim the other parameters be-
sides offset voltage as illustrated in Figure 10 The bias-cur-
rent trim relies on the fact that the voltage on the shield
driver and gain setting pins is one diode drop respectively
above and below the input voltage Input bias current can
be held to within 100 pA over the entire common-mode
range and input offset current always stays under 30 pA
The CMRR trims use the shield driver pins to drive the V_OS
adjust pins thus maintaining the LM363’s ultra-high input
impedance
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