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OP37AZ Datasheet(PDF) 11 Page - Analog Devices |
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OP37AZ Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page REV. A OP37 –11– APPLICATIONS INFORMATION OP37 Series units may be inserted directly into 725 and OP07 sockets with or without removal of external compensation or nulling components. Additionally, the OP37 may be fitted to unnulled 741type sockets; however, if conventional 741 nulling circuitry is in use, it should be modified or removed to ensure correct OP37 operation. OP37 offset voltage may be nulled to zero (or other desired setting) using a potentiometer (see offset nulling circuit). The OP37 provides stable operation with load capacitances of up to 1000 pF and ±10 V swings; larger capacitances should be decoupled with a 50 Ω resistor inside the feedback loop. Closed loop gain must be at least five. For closed loop gain between five to ten, the designer should consider both the OP27 and the OP37. For gains above ten, the OP37 has a clear advantage over the unity stable OP27. Thermoelectric voltages generated by dissimilar metals at the input terminal contacts can degrade the drift performance. Best operation will be obtained when both input contacts are main- tained at the same temperature. 10k RP OP37 V+ OUTPUT V– + – Figure 1. Offset Nulling Circuit Offset Voltage Adjustment The input offset voltage of the OP37 is trimmed at wafer level. However, if further adjustment of VOS is necessary, a 10 k Ω trim potentiometer may be used. TCVOS is not degraded (see offset nulling circuit). Other potentiometer values from 1 k Ω to 1 MΩ can be used with a slight degradation (0.1 µV/°C to 0.2 µV/°C) of TCVOS. Trimming to a value other than zero creates a drift of approximately (VOS/300) µV/°C. For example, the change in TCVOS will be 0.33 µV/°C if VOS is adjusted to 100 µV. The offset voltage adjustment range with a 10 k Ω potentiometer is ±4 mV. If smaller adjustment range is required, the nulling sensitivity can be reduced by using a smaller pot in conjunction with fixed resistors. For example, the network below will have a ±280 µV adjustment range. 1 8 4.7k 4.7k 1k POT V+ Figure 2. TBD OP37 –18V +18V Figure 3. Burn-In Circuit Noise Measurements To measure the 80 nV peak-to-peak noise specification of the OP37 in the 0.1 Hz to 10 Hz range, the following precautions must be observed: • The device has to be warmed-up forat least five minutes. As shown in the warm-up drift curve, the offset voltage typically changes 4 µV due to increasing chip temperature after power up. In the ten second measurement interval, these temperature- induced effects can exceed tens of nanovolts. • For similar reasons, the device has to be well-shielded from air currents. Shielding minimizes thermocouple effects. • Sudden motion in the vicinity of the device can also “feedthrough” to increase the observed noise. • The test time to measure 0.1 Hz to l0 Hz noise should not exceed 10 seconds. As shown in the noise-tester frequency response curve, the 0.1 Hz corner is defined by only one zero. The test time of ten seconds acts as an additional zero to eliminate noise contributions from the frequency band below 0.1 Hz. • A noise-voltage-density test is recommended when measuring noise on a large number of units. A 10 Hz noise-voltage-density measurement will correlate well with a 0.1 Hz-to-10 Hz peak-to-peak noise reading, since both results are determined by the white noise and the location of the 1/f corner frequency. Optimizing Linearity Best linearity will be obtained by designing for the minimum output current required for the application. High gain and excellent linearity can be achieved by operating the op amp with a peak output current of less than ±10 mA. Instrumentation Amplifier A three-op-amp instrumentation amplifier provides high gain and wide bandwidth. The input noise of the circuit below is 4.9 nV/ √Hz. The gain of the input stage is set at 25 and the gain of the second stage is 40; overall gain is 1000. The amplifier bandwidth of 800 kHz is extraordinarily good for a precision instrumentation amplifier. Set to a gain of 1000, this yields a gain bandwidth product of 800 MHz. The full-power bandwidth for a 20 V p-p output is 250 kHz. Potentiometer R7 provides quadrature trimming to optimize the instrumentation amplifier’s ac common- mode rejection. R7 100k C1 100pF R1 5k 0.1% R3 390 R2 100 R4 5k 0.1% INPUT (+) INPUT (–) R5 500 0.1% R6 500 0.1% R8 20k 0.1% R9 19.8k R10 500 VOUT NOTES: TRIM R2 FOR AVCL = 1000 TRIM R10 FOR dc CMRR TRIM R7 FOR MINIMUM V OUT AT V CM = 20V p-p, 10kHz OP37 + – OP37 + – OP37 Figure 4a. TBD |
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