LM363 datasheet(13/22 Pages) NSC

13 / 22 page

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

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

TLH5609 – 15

FIGURE 7 Output Clamp

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

andor 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

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 VOS

and CMRR While the shield drivers can sink several mA

VOS 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

TLH5609 – 16

FIGURE 8 Driving Shielded Cables

MISCELLANEOUS TRIMMING

The VOS 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 VOS

adjust pins thus maintaining the LM363’s ultra-high input

impedance

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