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FEATURES Functionally Complete Precision Conditioner High Accuracy Low Input Offset Tempco: 0.1 V/ C Low Nonlinearity: 0.025% High CMR: 160 dB (60 Hz, G = 1000 V/V) High CMV Isolation: 1500 V rms Continuous 240 V rms Input Protection Small Package: 1.0" 2.1" 0.35" DIP Isolated Power Low-Pass Filter (f C = 3 Hz) Pin Compatible with 1B41 Isolated RTD Conditioner APPLICATIONS Multichannel Thermocouple Temperature Measurement Low Level Data Acquisition Systems Industrial Measurement & Control Systems
Isolated mV/Thermocouple Signal Conditioner 1B51
FUNCTIONAL BLOCK DIAGRAM
Overall NMR is 60 dB and CMR is 160 dB min @ 60 Hz, G = 1000. The 1B51 is specified over -25C to +85C and operates over the industrial (-40C to +85C) temperature range.
GENERAL DESCRIPTION
The 1B51 is a precision, mV/thermocouple signal conditioner that incorporates a circuit design utilizing transformer based isolation and automated surface mount manufacturing technology. It provides an unbeatable combination of versatility and performance in a compact plastic package. Designed for measurement and control applications, it is specially suited for harsh environments with extremely high common-mode interference. Unlike costlier solutions that require separate dc/dc converters, each 1B51 generates its own input side power, providing true, low cost channel-to-channel isolation. Functionally, the signal conditioner consists of three basic sections: chopper stabilized amplifier, isolation and output filter. The chopper amplifier features a highly stable offset tempco of 0.1 V/C and resistor programmable gains from 2 to 1000. Wide range zero suppression can be implemented at this stage. The isolation section has complete input to output galvanic isolation of 1500 V rms continuous using transformer coupling techniques. Isolated power of 2 mA at 6.2 V is provided for ancillary circuits such as zero suppression and open-input detection. Filtering at 3 Hz is implemented by a passive antialiasing filter at the front end and a two-pole active filter at the output.
DESIGN FEATURES AND USER BENEFITS
High Noise Rejection: The combination of a chopper stabilized front end with a low-pass filter provides high system accuracy in harsh industrial environments as well as excellent rejection of 50 Hz/60 Hz noise. Input Protection: The input is internally protected against continuous application of 240 V rms. Low Cost: The 1B51 offers a very low cost per channel for high performance, isolated, low level signal conditioners. Wide Range Zero Suppression: This input referred function is a convenient way to null large input offsets. Low-Pass Filter: The three pole active filter (fC = 3 Hz) reduces 60 Hz noise and aliasing errors. Small Size: The 1B51 package size (1.0" x 2.1" x 0.35") and functional completeness make it an excellent choice in systems with limited board space and clearance.
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Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 Fax: 617/326-8703
1B51-SPECIFICATIONS (typical at +25 C and V =
S
15 V unless otherwise noted)
OUTLINE DIMENSIONS
Model GAIN Gain Equation Gain Error Gain Temperature Coefficient 1 Gain Nonlinearity OFFSET VOLTAGES Input Offset Voltage Initial, @ +25C (Adjustable to Zero) vs. Temperature vs. Time, Noncumulative Output Offset Voltage Initial vs. Temperature INPUT OFFSET CURRENT Initial vs. Temperature INPUT BIAS CURRENT Initial @ +25C vs. Temperature INPUT IMPEDANCE Power On Power Off INPUT VOLTAGE RANGE Linear Differential Input Max CMV, Input to Output AC, 60 Hz, Continuous Continuous, DC CMR @ 60 Hz, 1 k Source Imbalance, G = 1000 NMR @ 60 Hz Transient Protection INPUT NOISE Voltage, 0.1 Hz to 10 Hz, 1 k Source Imbalance RATED OUTPUT Voltage, 2 k Load, min Current Output Noise, DC to 100 kHz Impedance, DC FREQUENCY RESPONSE Bandwidth, -3 dB ISOLATED POWER Voltage, No Load Current Regulation, No Load to Full Load Ripple POWER SUPPLY Voltage, Rated Performance Voltage, Operating Current, Quiescent PSRR ENVIRONMENTAL Temperature Range Rated Performance Operating Storage Relative Humidity CASE SIZE
1B51AN
1B51BN
RFB RG
G = 1+

x2
Dimensions shown in inches and (mm). * * * 0.025% (0.04% max)
1% max 50 ppm/C 0.035% ( 0.05% max)
25 V (100 V max) 0.1 V/C ( 0.5 V/C max) 1 V/month max -50 mV -175 V/C 0.6 nA (2.5 nA max) 2.5 pA/C (12.5 pA/C max) 10 nA 10 pA/C 50 M 40 k min 10 mV to 5 V 1500 V rms 2000 V 160 dB min 60 dB min IEEE-STD 472 (SWC) 1 V p-p 10 V 5 mA 1 mV p-p 0.1 dc to 3 Hz 6.2 V 5% 2 mA 7.5% 250 mV p-p 15 V dc 13.5 V to 18 V +12 mA @ +15 V, -4 mA @ -15 V 0.1%/V
* * * -25 mV -50 V/C * * * * * * * * * * * * *
PIN DESIGNATIONS
Pin
* * * * * * * * * * * * *
Designation HI PROT HI ICOM LO +15 V -15 V VO GND -VISO +VISO GAIN FB
11 12 14 15 16 17 22 23 34 35 37 38
-25C to +85C -40C to +85C -40C to +85C 0 to 95%, @ +60C 1.00" x 2.10" x 0.35" (25.4 x 53.3 x 8.9) mm
* * * * *
NOTES *Specifications same as 1B51AN. 1 See graph in text. Specifications subject to change without notice.
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1B51
Functional Block Diagram
INSIDE THE 1B51 USING THE 1B51
Referring to the functional block diagram, the 15 V power inputs provide power to both the output side circuitry and the power oscillator. The 25 kHz power oscillator provides the timing information for the signal demodulator and drives power transformer T2 for the input side power supplies. The secondary winding of T2 is half wave rectified and filtered to create the input side bipolar unregulated supplies. The signal input (HI) is single-pole filtered for noise rejection and antialiasing. The protection clamps limit the voltage at PROT HI to 8 V. Thus, a large voltage applied between HI and input common (ICOM) appears mostly across the input resistor. The chopper stabilized gain stage amplifies the differential input voltage with a gain set by external resistors. The voltage at the inverting input of the chopper stabilized amplifier (LO) should be equal to the input voltage at which the desired output voltage is zero. This is a true input referred zero suppression function. The signal is amplitude modulated onto a 25 kHz carrier and passed through the signal transformer T1. The synchronous demodulator restores the signal to the baseband. A two-pole active low pass stage filters out clock noise and completes a three-pole Butterworth filter formed with the input pole.
Gain Setting The gain of the 1B51 is controlled on the input side by a pair of user provided resistors (see Figure 1). A feedback resistor of between 10 k and 20 k is required between the feedback pin (FB) and the gain pin. The gain setting resistor is connected between the gain pin and input side common (ICOM). The gain equation is
R G = 1+ FB x 2 RG
Gains of 2-1000 can be achieved by adjusting this ratio. The accuracy of the resistor values must be taken into account when calculating the initial gain accuracy of an application. The initial accuracy of the 1B51 must then be added to the resistor errors to predict the total accuracy. Likewise, the ratiometric temperature coefficient of the gain and feedback resistors must be added to the temperature coefficient of the 1B51 to predict the total resulting thermal drift. It is possible to use a trimming potentiometer to correct for initial gain and system gain errors. The feedback resistor can be comprised of a resistor in series with a trimming potentiometer, as long as the total resistance remains between 10 k and 20 k. Alternatively, the gain resistor can also be an adjustable resistor. In general, the greater the trim range, the coarser the resolution. Zero Suppression Since the 1B51 is a differential input device, true input referred zero suppression can be accomplished (see Figure 1). A voltage reference powered by the input side power supplies is applied to the LO terminal. Since the transfer function is VO = (V(HI )-V(LO))xGAIN the input voltage for which the desired output is zero should be applied to the LO pin. The equation is
V Z =1.25(R2 /(R1 + R2 ))
Figure 1. Input Gain Setting and Zero Suppression
Any drift of this input zero suppression voltage appears as offset drift, so a temperature stable reference should be used. The source impedance at the LO terminal should be kept below 1 k.
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1B51
Open Input Detection The 1B51 can sense an open thermocouple or broken input line with the addition of an external resistor. By connecting a 220 M resistor between the HI pin and the positive or negative isolated supply, an open input will cause a positive or negative full-scale output, respectively. To preserve the normal mode input protection capability of the 1B51, the resistor must be able to withstand 220 V ac. A high voltage rating can be obtained by connecting lower value resistors in series. Cold Junction Compensation When using a thermocouple as an input to the 1B51, a second thermocouple junction is formed at the terminations of the thermocouple wires, commonly referred to as the cold junction. The measured output voltage of the sensor is the voltage generated by the thermocouple minus the voltage generated by the cold junction. Since thermocouples are specified with 0 V representing 0C, it would be ideal to maintain the cold junction at 0C. A more practical approach involves adding a temperature dependent voltage to the thermocouple signal so as to oppose the cold junction effects. This type of correction is known as cold junction compensation. Many different methods are commonly used to implement cold junction compensation. Usually a thermistor or a semiconductor sensor is used to generate the cold junction voltage. The slope
Figure 2. 1B51 Cold Junction Compensation
of the cold junction voltage must be the same as that of the thermocouple. Therefore, the cold junction compensation depends on the thermocouple type. Sometimes, one cold junction compensation sensor is used by a number of thermocouple channels. This is accomplished by measuring the temperature of the connection block directly, and adding the appropriate voltage to each uncompensated thermocouple channel after the gain has been taken. In all cases, the cold junction sensor must be in the thermal proximity with the connection block. Figure 2 shows a monolithic cold junction compensation device used with the 1B51. The Analog Devices AC1226 measures the ambient temperature and generates the appropriate cold junction voltage for several different thermocouple types.
TYPICAL PERFORMANCE CURVES
(@TA = +25 C, VS =
15 V)
+VIso Ripple vs. Capacitance
+VIso vs. Load
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PRINTED IN U.S.A.
Gain vs. Temperature
CMR vs. Gain
C1147-10-1/89

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