ANALOG DEVICES Economy, Guaranteed Low-Noise Chopper Stabilized Amplifier FEATURES Low Cost Ultra-Low Noise: 0.5uV p-p, 1Hz BW [2uV, max) Very Low Drift: 0.1uV/C max, 0.5pA/C max (235L) Excellent Long Term Stability: 5uV/yr Low Profile: 0.5" Height APPLICATIONS Precision Integrator Picoamp Current Measurements Microvolt Voltage Measurements Bridge Amplifier Balance Scales and Weighing Instruments GENERAL DESCRIPTION Analog Devices model 235 is a chopper stabilized inverting op amp that delivers premium performance at economy prices. It is pin-compatible with existing, more expensive modules, allow- ing designers to upgrade systems while realizing significant cost savings. Foremost among model 235s electrical specifications is its outstanding noise performance of 0.5uV p-p (f = 0.01 to 1Hz) with a guaranteed maximum limit of 2uV in a 1Hz BW. Low voltage noise combined with low current noise (10pA p-p, 1Hz BW) yields low input noise for source impednaces up to sev- eral hundred-thousand ohms. The 235 also offers low voltage and current drift as a funtion of both temperature (0.14uV/C max, 0.5pAC max, 235L) and time (5uV/yr). This combina- tion of noise and drift performance makes model 235 ideally suited for demanding applications such as balance scales and weighing instruments requiring high accuracy and excellent long-term stability without the use of front panel balance pots or periodic internal adjustment. Model 235 has been designed to virtually eliminate intermodu- lation problems caused by beating against power line frequen- cies. The choppers ultra-stable oscillator is precisely set at the factory to a frequency that minimizes interactions with har- monics of 50, 60 and 400Hz power lines. For new and upgraded designs, model 235 sets the benchmark for economy chopper performance. APPLICATIONS The model 235 inverting amplifier should be considered when long term stability must be maintained with time and tempera- ture, and wherever maintenance-free operation of instruments 6.01 THz VOLTAGE NOISE and remote circuits is essential. Typical applications include amplification of microvolt signals, precision integration and analog computing. Low input noise and stable offset voltages make model 235 an ideal preamp for precision low frequency applications such as DVMs, 12 to 16 bit A to D converters, and error amplifiers in servo and null detector systems. GUARANTEED NOISE PERFORMANCE The excellent 1Hz voltage noise performance of model 235 (Figure 1) results from careful selection of critical design com- ponents during manufacturing. Selection permits 1Hz voltage noise to be maintained very near the typical specification (O0.5uV p-p). Vn, MODEL 235: 0.01 1Hz tuVv { Vn, MODEL 233 0.01 1Hz In, MODEL 235 0.01 1Hz SpA { Figure 1. Model 235 Voltage and Current Noise. Model 233 Voltage Noise Shown for Comparison OPERATIONAL AMPLIFIERS 8&5SPECIFICATIONS (typical @ +25C and +15V unless otherwise noted) MODEL 235J 235K 235L OPEN LOOP GAIN DC ,2k ohm load 5x10V/V min * * RATED OUTPUT Voltage +10V min * * 0S Current +5mA min * * (12.7) Load Capacitance Range 0.01NF * * FREQUENCY Unity Gain, Small Signal iMHz * * Full Power Response 5kHz min * * Slew Rate 0.3V/us min * * Overload Recovery 10 sec * * INPUT OFFSET VOLTAGE Initial Offset,! @ +25C +25uV max +25uV max 15uV max vs. Temp, 0 to +70C +0.5uV/C max +0.25uV/C max +0.1nV/C max vs. Supply Voltage +0.1uV/% * * vs. Time t5uV/year * * vs. Turn On, 10 sec to 10 min +3uV * * INPUT BIAS CURRENT Initial, @ +25C +100pA max +50pA max +50pA max vs. Temp, 0 to +70C +1pA/C max 0.5pA/C max +0.5pA/C max * vs. Supply Voltage 0.2pA/% * INPUT IMPEDANCE Inverting Input to 300k ohns * * Signal Ground INPUT NOISE 0.01 to 1Hz, typ 0.5uV p-p 0.01 to 1Hz, max 2pV p-p 2uV p-p 0.1 to 10Hz 3.5UV p-p * * 10Hz to 10kHz 5yuV rms * * Current, 0.01 to 1Hz 10pA p-p * * 0.1 to 10Hz 30pA p-p * * INPUT VOLTAGE RANGE (-) Input to Signal Ground 415V max * * POWER SUPPLY (V dc)? Rated Performance Operating +15V @5mA * +(12 to 18)V * TEMPERATURE RANGE Rated Specifications Operating Storage 0 to +70C * -25C to #85 C * -55C to +125C * *Specifications same as model 235J. ' Externally adjustable to zero. ? Recommended power supply, Analog Devices model 904, +15V dc @ 50mA Specifications subject to change without notice. 86 OPERATIONAL AMPLIFIERS OUTLINE DIMENSIONS Dimensions shown in inches and (mm) 1.51 MAX -- (38.1) >| U | 0.20(5.1} MIN 0.25 (6.3) MAX U =| |=-0.040 DIA. (1.1) | MAX (38.1) 1.51 K- we| |< 0.10 GRID BOTTOM VIEW (2.5) NOTES: *Optionat Trim Pot Analog Devices Model 79PR50k Connect Trim Terminal to Common if Trim Pot is not used. 1. SG Should Be Tied to Common. 2. Mating Socket AC1010 3. Weight: 27 grams. -20 -40 PHASE SHIFT -6) -80 -100 120 OPEN LOOP GAIN - dB -140 OPEN LOOP PHASE SHIFT Degrees = o a 10 10 10 FREQUENCY -- Hz 10 1 Figure 2. Open Loop Gain and Phase Shift vs. FrequencyModel 235s low frequency noise characteristics complement its very low drift performance to yield exemplary stability and accuracy for a chopper stabilized amplifier. A plot of noise as a function of bandwidth for model 235 is shown in Figure 3. 100 = o *NOISE MEASURED FROM 0.01Hz INPUT VOLTAGE NOISE 4V p-p 3 1 1 10 100 BANDWIDTH Hz Figure 3. Input Voltage Noise vs. Bandwidth for Model 235 INPUT IMPEDANCE CONSIDERATIONS Maximum input impedance for inverting amplifiers of all types is limited by bias current, bias current drift, and noise current. These currents flowing through the source impedance increase total error and noise as the input impedance increases. Figure 4 is a plot of total offset voltage, voltage drift and noise vs. input resistance for the model 235. 100 235J OF FSET vy 235L OFFSET {uV) 235K OFFSET (uV) - s 2353 VOLTAGE DRIFT (xV/PC) 0.1 1Hz NOISE {uV p-p) BV p-poruV/C INPUT VOLTAGE: NOISE, DRIFT, OFFSET 235L VOLTAGE DRIFT (uV/C) ~~~. 235K VOLTAGE DRIFT (uV/C) 10 ye 10 Rin ohms Figure 4. Uncompensated Offset, Drift and Noise vs. Ripy INITIAL OFFSET ADJUSTMENT Model 235 has low, untrimmed offset voltage specifications of 25uV max for the J and K versions, and 15uV max for the L version. In many applications there will be no need to further trim the offset. In such cases the trim terminal may either be left open, or grounded. If voltage offset adjustment is desired, it may be done with a potentiometer or selected fixed resistor network, as shown in the outline drawing. For circuits where the total input and source resistance remain relatively constant, the entire offset may be zeroed out with the voltage offset adjustment. The circuit of Figure 5 should be used to compensate for bias current offset when using the model 235 as a current to voltage converter. The potentiometer-resistor network provides a com- pensating bias current to cancel the amplifiers own input bias current. When the amplifier is used with a widely varying input resis- tance and minimum offset is desired, the voltage and current trim potentiometers should be used. The voltage offset should be zeroed with a low value (e.g. 1k ohm) resistor connected from the inverting input to ground. The offset current adjust- ment should be made with the maximum expected value of Rj connected between the input and ground. lin Ry Ri 7 Vs ao Ein | - 235 > Eout = (R/Ri) Ein VOLTAGE OFFSET ADJUST -V, (50k22 pot) oat) wu! | CURRENT OFFSET ADJUST {50k2 pot) v Figure 5. Offset Current Voltage Cancellation Tks INVERTING OPERATION The model 235 is designed for use in the inverting mode. It is important that the SG (equivalent to +in) terminal be kept at the same potential as the amplifiers common terminal. Any voltage difference between these points is similar to a common mode voltage, and performance cannot be guaranteed under such conditions. The model 235 is also an excellent amplifier for measurement and conversion of low level current sources to proportionate voltages. With offset current externally zeroed, input currents of ten to twenty picoamperes can be amplified and converted to a voltage source for further processing. SHIELDING, PICKUP AND GROUNDS Model 235 has an internal electrostatic shield that prevents pickup of extraneous signals and radiation of chopper noise by the module. One precaution is to insure that noise sources are shielded from the inverting input. The user should also insure that ground loops do not occur which can add extraneous sig- nals when amplifying from microvolt or millivolt sources. Ground loop errors most often occur when the power supply current is allowed to flow through the input (signal) ground connection. When this happens, a voltage drop is developed across the power supply leads which appears as a voltage gen- erator in series with the signal source, and as error at the out- put. This is effectively eliminated by insuring that the signal return lead does not carry the power supply current. Figure 6 illustrates the proper connection. Another source of error is the small voltages developed by the junction of dissimilar metals encountered in the external con- nections to the amplifier. Normally insignificant, these therm- OPERATIONAL AMPLIFIERS 87ocouple effects may approach the magnitude of the drift specifications of the 235. Careful attention to interconnection layout design will minimize these errors. fo | sit SIGNAL 7 Re RETURN lL LEAD | LOAD RETURN POWER SUPPLY COMMON * SIGNAL AETURN ANDO LOAD RETURN SHOULD BE CONNECTED TO POWER COMMON AS CLOSE TO AMPLIFIER PINS AS POSSIBLE Figure 6. Ground Connection INTERMODULATION CONSIDERATIONS If noise at medium frequencies finds its way into the input cir- cuits of carrier amplifiers (chopper amplifiers and the chopper- stabilizing portions of chopper-stabilized amplifiers), it may beat with the chopper frequency and produce sum and dif- ference frequencies. The sum and noise are unimportant, because they are usually filtered out. But the difference fre- quencies (which can include dc) usually interfere directly with the low-level low-frequency signal information. These effects can be examined with an oscilloscope. Model 235 employs specially designed internal shielding and a stable, factory-set oscillator frequency to drastically reduce problems caused by interference from 50, 60 and 400Hz power lines. The user can take further precautions to eliminate inter- modulation problems by: 1. Properly shielding input and power supply leads. 2. Using shielded supplies with low ripple and source imped- ance at the line harmonic frequencies. 3. Avoiding ground loops and locating the amplifier far from interference sources. THE T NETWORK High gains and high input impedance to an inverting amplifier normally require excessively large feedback resistors. Such a resistor is relatively expensive, particularly for low tolerance units. Furthermore, any stray capacitance across this single resistor significantly reduces bandwidth. The T network in Figure 7 minimizes these problems. If R/Rj is at least 5:1, there will be no measurable change in other performance char- acteristics. If the ratio is lower, the effective drift and noise gain will be increased compared to the signal gain. As a general rule, make the ratio R/Rj approximately equal to R2/R1. This results in reasonable values of resistance for Rg, and a minimal 88 OPERATIONAL AMPLIFIERS (-} Ein OW PN Rj 3 Rg OE, | Rp +iRq i Roi |] Ro t'Ryk Rp) (sc) | Ry Ro fo" OR ta Pe i i com Fo La lil ay V Ip Ry oR, Figure 7. T Network increase in noise and gain drift compared to the standard two resistor circuit. The T network also permits gain to be varied, by changing Rj, without the necessity of connecting a switch or potentiometer directly to the highly sensitive inverting input termina... OVERLOAD RECOVERY The overload recovery circuit shown in Figure 8 will prevent the input circuitry from becoming saturated. This circuit, con- nected externally, will allow the amplifier to recover from over- load in less than 0.5us. Without this circuit overload recovery will require up to 10 seconds. feast RECOVERY 15k 1 | clRCUIT | -15V | +4o--} of | ~ 1A insets | | b FD333 | . IN9618 | HAs, ofe | | , a s1sy | fk tk Ry lin Pin 1k MN. i 2in 0 >My ce . s +t 3G bO Sout USE IF OVERLOAD CURRENTS 4p-- 0-4 aye IN EXCESS OF AMPLIFIER { FO i va OUTPUT CURRENT CAPABILITY ME 353 s MAY BE ANTICIPATED baaape--! Figure 8. Overload Recovery Circuit MODEL 234 WIDEBAND CHOPPER The forte of model 235 is low frequency applications requiring high accuracy and highest stability. For wideband designs, the model 234 chopper is the recommended amplifier. The 234 has very good drift specifications coupled with outstanding bandwidth (2.5MHz unity gain, 500kHz full power) and wide- band noise performance (1.5u'V p-p, f = 0.1 to 10Hz and 2u'V rms, f = 10 to 10kHz). Model 234 is preferred for designs of wideband microvolt sig- nal processors, low duty cycle pulse train integrators and fast analog computers.