Model 1815 - Keithley 181 Service Manual

THEORY OF OPERATION
MODEL 181
rnugl~yiblo, The bootstrap supply is decoupled from the input
via C414. C415, R425and R426. Rejection from the + 15V sups
plies is critic;iil as well. Therefore. current regulators CR406
and CR407 are a necessity iRp> 1M ohm) since Rs of VR403
and VR404 is approximately 100 ohms,
The key to maintaimng low current noise in the drain circuit of
Cl413 IS the selection of a low noise, high gain device for Q414
and maintaining low emitter and base impedances over the
bandwidth, R431, R432 and R433 provide bias for Q414 and
are referer~:ed 10 _t VR for low noise contribution
Noise contribution from the second stage depends upon drain
loads of Q413, gain of the lirst stage and input noise of the second
srage~ Q412 is a precisely matched low noise amplifier Wan
sister piiir which act as the second stage for the 181 preamp,
III addilior 10 low now, Ihls fransislor pa~r IQ4121 is required
because of 1,s hlgtl CMRR performance.
4.18. Linearity and Gain
,r a diifarential DC amplifier. nonlinearities irI the input stage
provide Ihe Iirnifal,on 10 gain linearity under the closed loop
condluon. In fact, no amount of loop gain car cailcol these
nonIinearities. The input FETS are square law devices which
means the output current varies as the square of the VGS. In
order fo r:anr:o, this nonlinearity. a second FET is used as a
load 10 the Hurst FET idifierer,lIaI pair). The degree of linearity
depends upon the match,ng between the two devlces~
The lolaI amplifier loop gain must be kept high so that closed
Ioop gain is independent of open loop gain. This eliminates
problems with gain drifts due to time/temperature var~atlons
inside Ihe loop. To maintain stable closed loop gains of 1000
i2mV rangel. 100 IZOmV range1 and 10 (200mV range) apt
propriate film or wirewound resistors are chosen as feedback
elements. Refer to Table 4~2~
TABLE 4-2
GAIN RESlSTORS
Range Gain Resistors Gain Adjustment
2mv R40S. R414. R435 1000
R40S
ZOnl" R410, R415, R435 10" R410
zoomv R411, R416. R434
10 R411
Thermal drifts in the input amplifier are relatively unimporfant
since they are autozeroed our. Slowly varying drill come
ponents isuch as solder joints, reed relay contacts) are of little
significance. 'Therefore, no special lowthermal term~naIs are
necessary irl the feedback ,oop
However, for the nanovolt ranges (2mV. ZOmVl, it IS
necessary Lo use wirewound resistors wherever low voltage
signals are present, This is why R435 is a wirewound element,
whereas R434, R409-R416 are film resistors. Film resistors
display much higher voltage noise whereas wirewounds are
avaiInb,e whose noise approaches theoretical limifs. R435 is
s&x&d as 200 ohms since the Johnson noise of this value is
negligible compared to amplifier noise.
4.20. Offset Null
4.21. NMRR Filter and Buffer
Additional NMRR is provided 111 the preamp using a threepole
cascaded filter. As can be seen from Figure 4~7. the three
poles are provided by three RC networks~
Wher, FFT Q4,S turns o,. sii drw 0405. 0407 and Q4OS~ The
fliters arc mow in tile nrw11 arrd Ihe input signal is now liltered
and amplliicd, The filter settling I~me is equal 10 200
mi,Iiseconds. After lhe 200 millisecond ~rlferval the signa
integration 1s done by the A/D~ FET 0420 (now lums 011, and
FETs Q419. Q405, Q407 arid 0408 lurn off, Since 110 liller IS !n
while the amplifier looks at "zero," settling time depends on
amplifier speed and is 4 milliseconds. The zero is lhen Inn
tegrated by the A/D.
The reasorl the filter must shut off is so fhc A/D can record
amplifier xxo rlghl after if records sign;lI By keeping the tlrll~
delay bctween signal anti iclo as small as pilss~t~lo, effective
,,o,se ar,d dr,f, iire reduced tc lI,e desired lkwel. Co~lcurrerltly,
filtering of ~nrxmal mode signals IS accomplished only whet1
needed; wl,an looking at the signn,~
T11e first pole is formed by "416, C407 and Q4oS. The on
resistance of Q409 rnair~lains i:,osed loop gain aI all ire
qurncies greater than 10 which maintains ampI,firr stability~
The second pole is formed by C406. R406 and Q407. It is butt
fered from tile third pole by U401 5, The third pale is formed by
R401, C403 arid Q405~ AI, 11,rw ,x>Ies ,havr Ihr same 1,mc con
slant 01 10 miI,lseconds~ U401A builcrs Ihr: oulput 01 thn third
p0le.
Buffers U4OlA and U4OlB are boo&trapped in a similar
fashion to the input amplifier. VR401 and VR402 provide the
+6.2V reference, and emitter followers WI1 and Q402 supply
Ihe currenl 10 ,,4Ol, R40,C and R407D b,as Ihi! r~iemnces.
The effect oi This boofstrap is fo provide higher CMRR 10
"401, Ihnrefore imprrivirng linearity.
The baotstia,, CllClllt aoiis a pole I" 11k ir"qlicrrcy rcsp""sa 0,
U4016. The dominant pole formed by R407A. R407R. C401
arid C402 m~intnins stability rif the bootsrrap c~rcoif. Q403 and
Q404 illIi ,ciw capai:ilarK:~ FtTs used as d,odc clamps 10 Ihii
boolslraji c~rcuil to preve~lf latch up rcsult,ng lrom driviing
inputs to U4OlB out 01 lhoir common mode range durincj
overload
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