Precision
lsolation
Amplifier
ANALOG
DEVICES HighCMV/CMR,
t15VFloating
Power
FEATURES
VersatileOp Amp Front End: Inverting,Non-lnverting,
DifferentialApplications
Low Nonlinearity:0.025%max, Model277K
Low Input Offset Voltage Drift: 11tVl"Cmax, Model 277K
FloatingPowerOutput: tl5V @ t1SmA
H i g hC M R : 1 6 0 d 8m i n @d c
HighCMV: 3500vrms
APPLICATIONS
Programmable
Gain lsolatedAmplifier
lsolatedPowerSourceand Amplifier for BridgeMeasurements
I nstrumentation
Amplifier
InstrumentationGradeProcessSignallsolator
CurrentShuntMeasurements
GENERAL DESCRIPTION
Model 277 is a versatileisolationamplifier which combinesa
high-performance,
uncommittedoperationalamplifier front
end with a precision,isolatedoutput stageand a floating power
supply section.This configuration,shown in Figure L, makes
the 277 ideally suitedto insrrumentationapplicationswhere
the needfor variousforms of signalconditioning,high CMV
protection and isolatedtransducerpower requirementsare
encountered.
The input stageis a low drift (tlgV/'C max, model277K)
differential op amp that may be connectedfor use in inverting,
non-invertingand differential configurations.The circuitry
employedaround the operationalamplifier input stagecan be
designedby the userto suit eachapplication'sparticularsignal
processingneeds.A full t10V signalrangeis availableat the
output of the front end amplifier.
combined with the output stage'slow nonlinearity (O.05,/o,
models 2771/AandO.O25o/omod,el277K), these high CMR and
CMV ratings facilitate accurare measurements in the presence
of noisy electrical equipment such as morors and reliys. In
addition, model 277A offers a -25"C to +85"C rated operating
temperature range. All versions of model 277 have a t1O volt
ourput range.
The floating power supply section provides isolated 115 volt
outputs capable of delivering currents up to t15mA. This
feature permits mod,el277 to power transctucersand auxilliary
isolated circuitry, thereby eliminating the need for a separate
isolated DC/DC converter.
All of the features of the model 277 isol,attonamplifier are
packagedin a compact (3" x 2.2" x 0.59") module. As an
assuranceof high performance reliability, every model 277 is
factory tested for CMV rating by application of 35OoVrro,
(14900V peak) between input and output common terminals
for one minute (meets NEMA and CSA requirements for
660vnns service.) In addition, the 277 has a calculated MTBF
o f 1 33 , O 0 Oh o u r s .
OUT
Figure l. Model 277 Functional
Block Diagram
The isolated output stage includes a special modulator/demodulator technique which provides the 277 with 160dB minimum
DC common mode rejection between input and ourput common and an input-to-output CMV rating of 35OOVrr,,".When
Informationfurnishedby Analog Devicesis believedto be accurate P.O. Box 280; Norwood, Massachusetts02062
U.S.A.
and reliable.Horruever,
no responsibility
is assumedby AnalogDevices Telex: 92t1491
Cables:
ANALOG
NORWOODMASS
use;
for its
nor for any infringements
of patentsor other righti of third
partieswtrichmay resultfrom its use.No licenseis grantedby implication or otherwiseunderany patentor patentrightsof AnalogDevices.
SPECIFICATI0NS
(typicar
ttsvunress
at+2boc
and
otherwise
noted)
MODEL
2771
INPUT STAGE PERFORMANCE
OPEN LOOP GAIN
l06dB min
INPUT OFFSET VOLTAGE
Inirial, @ +25"C (Adjusrable ro Zero)
vs. Tempera!ure
Offst Untrimmed
Offst Trimmed to Zero
vs. Supply Voltage
vs. Time
INPUT BIAS CURRENT
lnitial, @ +25oC
vs- Temperature
vs- Supply Voltage
277K
277A
*
15pV/"C
OUTLINE DIMENSIONS
Dimensionsshownin inchesand (mm)
t1.5mV max
0.595MAX {15.1)
t5gV/oC max
!3!V/"C max
t3opv/V
t3.5pvlmo
tllrV/"C
max
0.20MrN (s.08)
0.25MAX (6.35)
3.O4MAX (77.21
120nAmd
t50pA/"c
t100pA/V
tnitial, @ +25'C
vs. Supply Voltage
t6nA
i5opA/v
INPUT IMPEDANCE
Differential
Common Mode3
4MQ
lo0MQll4pF
INPUT NOISE
Voltage,0.0lHz to 1oHz
lOHz to lkHz
Current.0.0lHz to 1oHz
1,rV p-p
SrrVrms
SspAp-p
INPUT VOLTAGE RANGE
Common Mode Voltage3 ^
common Mode Rejecrion', cMv = 110V, 6oHz
Max Safe Differentiel Voltage
ilOV min
r00dB
l13V
ISoLATED PowER ourPUT"
Voltage/current 2
Load Regulation (No Load
l15V @tl5mA
+O, -60A
Full Load)
Line Regulation
Ripple, Full Load
OUTPUT STAGE Pf, RFORMANCE
GAIN
Cain Error
vs. Temperature
Nonlineariry, t10V
VOLTACE RATINGS
Max CMV, Output Com/lnpur Com
AC,60Hz, I Minute
Nonrecurring Spike ((1 Second)
Peak AC or DC, Continuous
CMR, Output Com/lnput Coms
DC
6OHz
Leak. Cur., Input/Output I 15V*", 60Hz
a;:.T,^^
I
BOTTOMVIEW
WEIGHT:150gms
1V/V
tO.5% mar
t5oppm/oc ma*
aO.050,6
max
3500V*" max
15000V pk max
t2500V max
z d
160dB min
120d8 min
lllA rms max
fr2
3T
!1OmVmax
tloolv/'c max
tlmV/v
!1OO!V/mo
Volrage, 0.0lllz to 1oHz
lOHz to lkHz
POWER SUPPLY
Voltage, Rated Performance
Voltage, Operating
Current, Quiescent
I
=
o 3
lo,Qlll6pF
RATED OUTPUT
Vottage/Current
a l
O F
!50!V/oc
mu
11o0gv/'
c max
Figure 2. lnput StageGain, CMR and Phase
vs. Frequency
z.5kH?
1.5kHz
t10V min @ 15mA min
TttV p-p
25gV rms
-20
r15VDC
1 ( 1 4t o 1 6 ) V D C
+ 3 5 ,- 5 m A
t \
GAIN
"
-60
80L
100
9
o
12O' F
I
24O u
a
-25"c to +85"c
O to +7OoC
-25"c to +85"c
-55"c ro +85"c
NOTES:
'Cun
nt dr.wn from INPUT FEEDBACK
tcmind
mu$ b€ <smA.
rTotrl
drewn from IN FEEDBACK
oncnt
rnd cithcr +VISO or -VISO must bc <15il.
Itnput
+lN and {N tcminrls
comon
nodc spccific.dons
rc meNcd.t
with rcspcct to INPUT
'Protccrcd
for momcntry
shorts ro IN COM.
s
lehtion
mc.sr€d
cr INPUT COM wid rcsp€ct to OUT COM sd PWR COM.
+Gcificatio.s.rc
'Sp.cificcrions smc rs modcl277J.
to chsnge wirhout noiic..
{l
_ { 0
t . O "x 2 . 2 ' x O - 5 9 "
CASE SIZE
0
PHASE
0
Rated Performance
Operating
Storage
l-n'
J0mV p-p @ 70kHz
OUTPUT OFFSET VOLTAGE
Initial, @ +25'C (Adjustable to Zero)
vs. Temperature
vs. Supply Voltage
vs. Time
FREQUENCY RESPONSE
Small Signal, -3dB
Full Power,20V p-p Output
Settling Time 110V Srep ro 0.1%
I
MATING SOCKET_ AC1O53
max
ISOLATION IMPEDANCE
lnput Com/Output Com
Sp.crfic.rions $bjecr
t
2 INPUTFEEDBACK
I
:oo'
"
rk
10k
F R E O U E N C YH z
Figure 3. Output StageGain and Phase
vs. Frequency
COM
PERFORMANCE CHARACTERISTICS
Gain Nonlinearity: Nonlinearity error is expressed as a o/oof
peak-to-peak ourput voltage span; e.g. lO.O5o/o@ 1oV p-p output = t5mV max RTO nonlinearity error. Model 277 is available
in two maximum nonlinearity grades - 10.O5Yo(2771/A),
+o.o250/o(277K).
The nonlinearity of model 277 is virtually independent of output voltage swing. Therefore, the 272 can be used at any level
of gain and output signal range up ro t10V while maintaining
its excellent lineariry characteristics.
Output Voltage Noise: Peak-to-peak output voltage noise is
dependent on bandwidth, as shown in Figure 4. The graph
shows RTO noise, that is, output noise for a gain of 1VlV
through the isolator. For lowest noise performance, a low pass
filter at the output can be used to roll-off noise and undesired
signal frequencies beyond rhe bandwidth of interest. As gain
increases,voltage noise referred-to-input decreases,resulting in
higher input signal to noise ratios. The next section demonstrates how voltage noise, referred-to-input, can be calculated.
RTI noise, in a given bandwidth, (for Figure 8a) may be calculated as follows,
Ep (rms,RTI)=VSN''
where:
+ (EN'/G1)2
Ep, = total rms input stage voltage noise
E51, = rms output voltage noise (RTO)
Common Mode Rejection: A 160dB rejection of potential
differences between input and output common is achieved in
model 277 by maintaining low coupling capacitance berween
the input and output stages.Input-to-output rejection is a
function of frequency as shown in Figure 5 under the adverse
condition of lkQ in serieswith IN COM. CMR versus freouencr
for the input stage is shown in Figure 2.
The section on GUARDING TECHNIeUES & INTERCONNECTION demonstrates how to calculate total CMR error for
the isolator and indicates the precautions to be taken to preserve the modcl 277's inherently excellent CMR performance.
GUARDTNG TECHNTQUES & TNTERCONNECTTON
Model 277 CMR performance is best preserved by using shielded signal cable with the shield connected as close as possible to
signal low and IN COM to reduce pickup (see Figure 6).
I roo
F
;I
z
F 1 0
l
F
f
*RMS NOISE LEVELS CAN BE
APPROXIMATEDBY OIVIDIN(
THE PEAK.TO.PEAKNOISE
LEVELS BY A FACTOR OF 6.6
|t |t t t t l tl | t t t tr
F igure 4. o utput ;;;:;:;;
t
l
l
vs.Bandwidth
RTI Offset Voltage, Drift and Noise: Offset voltage, referred
to input (RTI) for model 277 may be computed by treating
the isolator as two cascadedamplifier stages.The input stage
has variable gain G 1 while the ourput isolation stage has a fixed
gain of 1- RTI offset is given by,
Eos (RTI) = Eosr + E65r/G1
where,
EOS1 = total input stage offset voltage
EoSz = outpur stage offset voltage
G1 = input stagegain
Overall CMR error at the output (e"rr) is due to the CMR of
the input amplifier and the CMR between input and output
stagesand is given by:
e*=
Offset voltage drift, RTI, may be calculated in the same manner.
where:
I
=
Figure 5. lnput-to-Output CMR vs.Frequency with lke
Source lmbalance
j=( G 'r') + - j I Q - CMRN'
CMR16
ecrn = input amp CMV with respect to IN COM
ero = CMV berween OUT COM and signal ground
CMRrlr = CMR of the input op amp
CMRro = CMR from input IN COM to OUT COM
Gt = input stagegain
To preserve CMR61 , amplifier source impedances should be
balanced with respect to IN COM. Components connected to
the input should be enclosed by a shield tied to IN COM to
reduce CMRI6 degradation due to unguarded capacitance to
ground.
High CMR16 is maintained with low capacitance between IN
COM and OUT COM. For best CMR performance, printed circuit layouts should minimize stray capacitance between input
and output stages.Do not run a ground plane under tbe isolatot
since tbis increasesinput-output coupling. CMR;6 also degrader
at high frequencies by resistance (RS) between IN COM and
signal ground. Voltage between OUT COM and source ground
divides between this resistance (generally wire resistance) and
the input-to-output capacitance resulting in an input error signal. If R5 becomes excessive,a capacitor from +IN to OUT
COM will help compensare for its effect on CMR. The capacitor
must withstand the isolation voltages encountered.
ADJUSTMENT PROCEDURE
The input and output offset voltage of model 277 canbe
trimmed as shown below with the isolator set up in the desired
circuit configuration.
( 1)
(2)
(3)
(4)
Refer to Figure 6 for terminal and component designations.
Connect IN COM to OUT COM and ser input signal to zero.
PlacefloatingDVM acrosslN FDBKandOUTPUT terminals.
Null DVM reading using output offset trim pore nriomerer
R6.
( 5 ) Disconnect IN COM from OUT COM.
( 6 ) Place DVM across IN FDBK and IN COM terminals.
( 7 ) Adjust input offset trim potentiometer, R1, until DVM
reads zero volts.
need for high quality post-amplifiers at rhe isolator ourput.
This network is extremely useful in wide dynamic range measurements such as flow, level or pressure where auto-gain ranging would be a desirable sysrem instrumentation fearure.
INPUT CONFIGURATION
Model 277's input stage is an isolated, uncommitted operational amplifier that may be configured to suit a variety of applications. Model 277 rr,ay be used in the same way as any op amp
except that the feedback is taken from the FDBK terminal
rather than the OUTPUT pin. Figure 8 shows four typical input
configurations for interfacing with a wide range of signal
sources.
(a) Non -I nverti ng Conf iguration
eo=re,ff+e**-*ff,
The overall gain of the isolator may be increased over a limited
range (5o/o)with a 5kS) porenriomerer connected between pins
1 Oa n d 1 2 .
APPLICATIONS
Programmable Gain Bridge Transducer Amplifier: The versatility of model 277 is shown by the programmable gain bridge
transducer amplifier application of Figure 7. In this circuit the
277's uncommitted front end and floating voltage ourpur permit both bridge excitation and signal gain conditioning to be
provided by the isolation amplifier.
(b) Summ i ng Conf iguration
"cu
-^+
Control switches are driven by TTL inputs which are isolated
from source ground by the opto-isolators in the control
switch. Control signals operate the CMOS switch network to
establish the gains shown in the table in Figure 7. The CMOS
switch network is operated in a manner that causesthe resistance of the switches only to be in serieswith the negative input of the isolator and not in serieswith the gain setting resistors. With this arrangement the switch resistance does not
affect gain accuracy. A resistor, Rg, should be in series with
-lN to reduce errors due to bias current drift.
With this circuit the isolator gain can be remotely set at a value
that optimizes input signal-to-noiseratio and eliminates the
^1
-^+
f--131....ry
|
Gz "r)Q
^t
$
i\GAIN
(-a
2T'
I
l our
+
<v
I
IN
coM
SOURCE
GROUND
R!
coM
LOAD
GROUND
(c) lsolated Differential Configuration
(d) Current SourceAmplif ier Configuration
Figure 8. Model 277 lnput Amplifier Conf igurations
ft".{ffiffi"-{;
Rr-eO-tgd+nq).
ReOfaG3>l1!(r
FOLATOi GAINt Gt
L@E
Yt'
:
'RGSlsroiflaED To
r$rffirz€ rfHJT OFF,
sT oxArreES$HE!a
Cl{AttlGlXG
GAFI-
Figure 7. ProgrammaNe Gain Bridge Tranducer Amplifier
a,
rir
,:
!0a$
K
q,
si
of.
,l;t
1:,
ffi
t,
!90