cS ANALOG Precision Isolation Amplifier DEVICES High CMV/CMR, *15V Floating Power FEATURES Versatile Op Amp Front End: Inverting, Non-Inverting, Differential Applications Low Nonlinearity: 0.025% max, Model 277K Low Input Offset Voltage Drift: 14V/C max, Model 277K Floating Power Supply: +15V de @ +15mA, High CMR: 160dB min @ dc High CMV: 3500V,m; APPLICATIONS Programmable Gain Isolated Amplifier Isolated Power Source and Amplifier for Bridge Measurements Instrumentation Amplifier Instrumentation Grade Process Signal Isolator Current Shunt Measurements GENERAL DESCRIPTION Model 277 is a versatile isolation amplifier which combines a high-performance, uncommitted operational amplifier front end with a precision, isolated output stage and a floating power supply section. This configuration, shown in Figure 1, makes the 277 ideally suited to instrumentation applications where the need for various forms of signal conditioning, high CMV protection and isolated transducer power requirements are encountered. The input stage is a low drift (t1uV/rC max. model 277K) differential op amp that may be connected for use in inverting, non-inverting and differential configurations. The circuitry employed around the operational amplifier input stage can be designed by the user to suit each application's particular signal processing needs. A full 10V signal range is available at the output of the front end amplifier. out OFFSET TRIM ouT GAIN TRIM rim (6)t TRIM ARM (8) - in Q) fT DEMOD FILTER +IN (4) a | as ~ + v OSCILLATOR eo eee ee 8 8 8 ITED Figure 1. Model 277 Functional Block Diagram The isolated output stage includes a special modulator/demodu- lator technique which provides the 277 with 160dB minimum de common inode rejection between input and output com- mon and an input-to-output CMV rating of 3500Vans. When combined with the output stages low nonlinearity (0.05%, models 277J/A and 0.025% model 277K), these high CMR and CMV ratings facilitate accurate measurements in the presence of noisy electrical equipment such as motors and relays. In addition, model 277A offers a -25C to +85C rated operating temperature range. All versions of model 277 have a +10 volt output range. The floating power supply section provides isolated +15 volt outputs capable of delivering currents up to +15mA. This feature permits model 277 to power transducers and auxilliary isolated circuitry, thereby eliminating the need for a separate isolated de/dce converter. All of the features of the model 277 isolation amplifier are packaged in a compact (3 x 2.2 x 0.59) module. As an assurance of high performance reliability, every model 277 is factory tested for CMV rating by application of 3500V ams (44900V peak) between input and output common terminals for one minute (meets NEMA and CSA requirements for 660Vims Service.) In addition, the 277 has a calculated MTBF of 133,000 hours. ISOLATION AMPLIFIERS 133SPECIFICATIONS (typical at +25C and +15V unless otherwise noted) DEL 277 277 Mo J K 217A OUTLINE DIMENSIONS INPUT STAGE PERFORMANCE !? . . , OPEN LOOP GAIN 106dB rin * * Dimensions shown in inches and (mm) INPUT OFFSET VOLTAGE Initial, @ $25C (Adjustable to Zero) ti.5mV nax * * 0.595 MAX (15.1) vs. Temperature Offset Untrimmed Offset Trimmed to Zero t5uvec max + 3uViC max 1pV/C max * 277 + a z < ~o a - 20 MIN (5,08) 25 MAX (6.35) : =~ 0.040 (1.02) U 028 3.04 MAX (77.2) | I = ISO INPUT FEEDBACK -4N +IN 13 Vs 14 vs. Supply Voltage +30nVIV vs. Time +3,5uV/ino * * INPUT BIAS CURRENT Initial, @ +25C 4+20nA max * * vs. Temperature t50pAPC * * vs. Supply Voltage 100pA/V * * INPUT DIFFERENCE CURRENT Initial, @ +25C +6nA * * vs. Supply Voltage +50pA/V * * INPUT IMPEDANCE Differential 4MQ * * Common Mode? 100M22|[4pF 2.23 MAX (56.64) INPUT NOISE ofetTrtToytToataTeytnry PWR COM 16 . BOTTOM VIEW | | 0.1 (2,54) GRID WEIGHT: 65 grams Voltage, O.O1Hz to 1OHz IV p-p * * 1OHz to TkHz 3uV rms * * Current, 0.01Hz to 10Mz 35pA p-p * * INPUT VOLTAGE RANGE Common Mode Voltage? 10V min * Common Mode Rejection , CMV = t10V, 60Hz 100dB * * Max Safe Differential Voltage t13V * * ISOLATED POWER SUPPLY Voltage/Current? Load Regulation (No Load Line Regulation Ripple, Full Load Full Load) 15V @ 15mA max +0, -6% 1V/V 30mV pp @ 70kHz MATING SOCKET AC1053 120 QUTPUT STAGE PERFORMANCE GAIN Gain Error vs. Temperature Nonlinearity, +10V Output V/V 40.5% Wax 50ppm/C max 0.05% imax 0.025% max VOLTAGE RATINGS Max CMV, Output Com/Input Com ac, 60Hz, 1 Minute Nonrecurring Spike (<1 Second) Peak ac or de, Continuous CMR, Output Com/Input Com de 60Hz Leak. Cur., Input/output 115V_,,., 60Hz rms? ISOLATION IMPEDANCE Input Com/Output Com 3500V ys max +5000V pk max +2500V max 160dB niin 120dB nun 1A ems max 10! QI): opF 100 . 8 x 1 = 80 By cMR (CMV oF x 2a a 60 * a . at OF OK zy 4 22 * Sz * N 3 OUTPUT OFFSET VOLTAGE 0.1 1 10 100 1k 10k 100k 1M Initial, @ +25C (Adjustable to Zero) +10mV nax * , FREQUENCY Hz vs. Temperature +100KV C max t50nV/C max 100nV/ C max vs. Supply Voltage t1mvV/\ * * 7 10004 * * . at _ Figure 2. Input Stage Gain, CMR and Phase FREQUENCY RESPONSE Small Signal, -3dB 2.5kHz * . vs. Frequency Full Power, 20V p-p Output 1.SkHz * * Settling Time 10V Step to 0.1% Ims * * RATED OUTPUT Voltage/Current 10V min @ 45mA min OUTPUT NOISE Voltage, 0.01Hz to 10Hz 7RV p-p * * v 3 10Hz to IkHz 25uV mais * * 2 POWER SUPPLY 320 1 Voltage, Rated Performance +15V de * * na t Voltage, Operating +(14 to 16)V de * * 7 3 Current, Quiescent 435, -5inA * * z 240 yw =z < TEMPERATURE RANGE S z Rated Performance Oto 47C . -25C to +85C 360' Operating -25C to +85C * * Storage -55C to 485C * * -80 ME Cy ES 7 * + 100 1k 10k 100k CASE SIZE 3.0" x 2.2" x 0.59 FREQUENCY Hz = Figure 3. Output Stage Gain and Phase NOTES: Current drawn fram INPUT FEEDBACK terminal must be <5mA. ? Total current drawn from IN FEEDBACK and either +Vygo OF -Vjgqy must be <15mA. Input common mode specifications are measured at +IN and -IN terminals with respect to INPUT COM. *Protected tor momentary shorts to IN COM. *solation specifications are measured at INPUT COM with respect to OUT COM and PWR COM, *Specifications same as model 277). Specifications subject to change without notice. 134 ISOLATION AMPLIFIERS vs. FrequencyPERFORMANCE CHARACTERISTICS Gain Nonlinearity: Nonlinearity error is expressed as a % of peak-to-peak output voltage span; e.g. t0.05% @ 10V p-p out- put = t5mV max RTO nonlinearity error, Model 277 is available in two maximum nonlinearity grades +0.05% (277J/A), 0.025% (277K). The nonlinearity of model 277 is virtually independent of out- put voltage swing. Therefore, the 277 can be used at any level of gain and output signal range up to 10V while maintaining its excellent linearity 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 1V/V 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 the bandwidth of interest. As gain increases, voltage noise referred-to-input decreases, resulting in higher input signal to noise ratios. The next section demon- strates how voltage noise, referred-to-input, can be calculated. 1000 /) "| % / cL > 1100 7 YA \Y il ew a UY $ & 10 2 \4 *RMS NOISE LEVELS CAN BE E a APPROXIMATED BY DIVIDING a a THE PEAK-TO-PEAK NOISE a LEVELS BY A FACTOR OF 6.6 1 10 100 1k 10k FREQUENCY Hz Figure 4. Output Voltage Noise vs. Bandwidth RTI Offset Voltage, Drift and Noise: Offset voltage, referred to input (RTI) for mode! 277 may be computed by treating the isolator as two cascaded amplifier stages. The input stage has variable gain Gy while the output isolation stage has a fixed gain of 1. RT] offset is given by: Eos (RTI) = Egg, + Eosy/G1 where: Egg, = total input stage offset voltage Eos, = output stage offset voltage (ay = input stage gain Offset voltage drift, RTI, may be calculated in the same manner. ae 140 7 T 120 j TEST CIRCUIT B0}-- in ronK <4 [we : ~ IN any ou com qe ces CMR ~ dB 60 fF [ os: ) & Pw COM t T = ie 5 O- 40. a mV SIGNAL SOURCE 20 | 1 10 100 Tk 10k FREQUENCY - Hz Figure 5. Input-to-Output CMR vs. Frequency with 1kQ. Source Imbalance RTI noise, in a given bandwidth, (for Figure 8a) may be caleu- lated as follows: EN (rms, RTD = EN, + (En, /Gy) where: En, = total rms input stage voltage noise En, = 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 between the input and output stages. Input-to-output rejection is a function of frequency as shown in Figure 5 under the adverse condition of 1kQ in series with IN COM. CMR versus frequency for the input stage is shown in Figure 2. The section on GUARDING TECHNIQUES & INTERCON- NECTION demonstrates how to calculate total CMR error for the isolator and indicates the precautions to be taken to pre- serve the model 277s inherently excellent CMR performance. GUARDING TECHNIQUES & INTERCONNECTION Model 277 CMR performance is best preserved by using shield- ed signal cable with the shield connected as close as possible to signal low and IN COM to reduce pickup (see Figure 6). MODEL 277 FLOATING INPUT SHIELD TRANSDUCER CABLE a ~s OUTPUT SHIFi.D TRANSDUCER SIGNAL 4 | tr A | | 77 | | | Ry | ' (ki Jk IN COM LOAD &) V/ Grouno FLOATING POWER =| (Rol [+ +2500 VDC MAX, CONTINUOUS 15V 15V hay TT + NOTES R POWER COM 1 GAIN- 1+ R (NON INVERTING CONFIGURATION} 1 2) OPTIONAL INPUT OFFSET TRIM (Ry) AND OPTIONAL OUTPUT OFFSET TRIM (Ro) ARE SHOWN: WHEN Ro AND R; ARE NOT USED LEAVE PINS 6, 7, 8, 13 OPEN Figure 6. Transducer, Power, Gain Resistors, and Shielding Interconnection Overall CMR error at the output (,,) is due to the CMR of the input amplifier and the CMR between input and output stages and is given by: where: gy = input amp CMV with respect to INCOM 39 = CMV between OUT COM and signal ground CMR = CMR of the input op amp CMRyio = CMR from input IN COM to OUT COM G, = input stage gain To preserve CMRyn, 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 CMRyjg degradation due to unguarded capacitance to ground. High CMRjo is maintained with low capacitance between IN COM and OUT COM. For best CMR performance, printed cir- cuit layouts should minimize stray capacitance between input and output stages. Do not run a ground plane under the isolator since this increases input-output coupling. CMRjg also degrades ISOLATION AMPLIFIERS 135at high frequencies by resistance (Rg) betwezn 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 sig- nal. If Rg becomes excessive, a capacitor from +IN to OUT COM will help compensate for its effect on CMR. The capacitor must withstand the isolation voltages encountered. ADJUSTMENT PROCEDURE The input and output offset voltage of mode! 277 can be 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 set input signal to zero. Place floating DVM across IN FDBK and OUTPUT terminals. Null DVM reading using output offset trim potentiometer Ro. Disconnect IN COM from OUT COM. Place DVM across IN FDBK and IN COM terrninals. Adjust input offset trim potentiometer, Ry, until DVM reads zero volts. (3) (6) (7) The overall gain of the isolator may be increased over a limited range (5%) with a 5kQ potentiometer connected between pins 10 and 12. APPLICATIONS Programmable Gain Bridge Transducer Amplifier: The versa- tility of model 277 is shown by the programmable gain bridge transducer amplifier application of Figure 7. In this circuit the 277s uncommitted front end and floating voltage output per- mit both bridge excitation and signal gain conditioning to be provided by the isolation amplifier. 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 tha causes the resis- tance of the switches only to be in series with the negative in- put of the isolator and not in series with the gain setting resis- tors. With this arrangement the switch resisiance does not affect gain accuracy. A resistor, Rg, should be in series with -IN 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-noise ratio and eliminates the need for high quality post-amplifiers at the isolator output. This network is extremely useful in wide dynamic range meas- urements such as flow, level or pressure where auto-gain rang- ing would be a desirable system instrumentation feature. INPUT CONFIGURATION Model 277s input stage is an isolated, uncommitted operation- al amplifier that may be configured to suit a variety of applica- tions. Model 277 may 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. SOURCE GROUND LOAD GROUND vn Vv (a) Non-Inverting Configuration Ry Re, .. Re leq ten " + e3 } Ri Re 3 RL SOURCE com , LOAD C ROUND GROUND (b) Summing Configuration OUT + 20 RL ab SOURCE com LoaD 7?! GROUND GROUND (c) Isolated Differential Configuration LOAD GROUND SOURCE /77 GROUND Vv (d) Current Source Amplifier Configuration Figure 8. Model 277 Input Amplifier Configurations +15V << 2k +7 5V ti-9 A FLOATING POWER Bans TO BRIDGE AND Sans AUXILLIARY CIRCUITS 75) <b 2k ! BRIDGE 18V - SOURCE GROUND __- \ \ && on RO. RO. on )\sineio eo = Gr en qev > atv (SEE TABLE) *lein <5V o*~CABLE RO. RO a Loab Rp = RQ - (909+ Ron), $5 VV GROUND 4+7.5V 9 Ron OF 405321102 928 ay CONTROL $ CONTROL CIRCUIT 1 2 cIRCUIT2 15 - : ISOLATOR GAIN, Gy t " vay rt Logic | Locic 9 sf $ INPUT. | INPUT Gr cmos a7ot gaat 3 909 4 L2 2N4275 T5V 10 SWITCH & AWA Ao FD333 a (CD4083AE) . + 2909 0 0 1 \ C 470 432 > AAA. AAA 1 9 10 7 14 Ww Wwe" 0 1 100 7 TIL 3 10 1 1 1000 111 it 6 4 24k CI ; > $ 3 4 is 7 2510 2 + RESISTORS USED TO t J J MINIMIZE INPUT OFF- SET CHANGES WHEN -18V 15 LOGIC INPUT LOGIC INPUT +75V -T5V CHANGING GAIN. Figure 7. Programmable Gain Bridge Transducer Amplifier 136 ISOLATION AMPLIFIERS