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The ☙0µV offset over the entire operating temperature range ensures a less than 2☌ temperature offset. The 1pA maximum IBIAS of the LTC6081 translates to a minuscule 0.05☌ temperature error with the 1MΩ input protection resistor. The 1MΩ resistors protect the circuit up to ☓50V with no phase reversal to amplifier output. Matching of R1 and R2 in Figure 2 is not important.įigure 5 shows the LTC6081 in a thermocouple amplifier. The advantage of the circuit in Figure 2 is that gain can be put at the front stage to ease the matching requirements of the second stage. This is true for all the above three circuits. For example, at gain of 10, to achieve 80dB CMRR, mismatch of R4/R3 and R6/R5 should be less than 0.1%. Where A V is the differential gain of the instrumentation amplifier. The above discussion assumes a perfect matching of R4/R3 and R6/R5. Difference amplifier with no input buffers < img src=' ' alt='Figure 4'> įigure 4. Even with a 1MΩ input resistor R3 or R5, the less than 1pA input bias current of LTC6081 will add less than 1µV to V OS. An obvious advantage of the LTC6081 is its super low input bias current. The impedance of the non-inverting and inverting inputs are R3 and R5 + R6, respectively. If the input resistance requirement can be eased, Figure 3 can be reduced to Figure 4, a single stage difference amplifier. As mentioned previously, it won’t have the high CMRR of the circuit in Figure 2 since we reduced the front stage gain to unity. This circuit can achieve rail-to-rail input range. To widen the input common mode range, the first stage gain can be reduced, but this will compromise CMRR performance.įigure 3 is a reduced circuit of Figure 2 with a unity gain buffer at the front stage.
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The larger the first stage gain or input differential signal is, the narrower the input common mode range is. Where V + and V – are the positive and negative supply voltage respectively. Assuming the differential and common mode input voltage are V IN(DM) and V IN(CM) respectively, the output voltages of op amp A and B are then V IN(CM) – (2R1/ R0)V IN(DM) and V IN(CM) + (2R1/R0)V IN(DM) respectively. One drawback of the circuit in Figure 2 is its common mode operating range is no longer rail-to-rail. Since a single LTC6081 op amp drifts less than 0.8µV/☌, the amplifier in Figure 2 will drift less than 1.1µV/☌. Statistically, the total V OS is √ 2 times the V OS of a single op amp. The input referred offset of the amplifier is Gain can be changed by simply changing R0 without affecting the resistor matching. Ratio matching of R4/R3 and R6/R5 is critical for CMRR.
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In this two stage structure, the differential voltage passes through the first gain stage with gain of 1 + 2 R1/R0 while the common mode voltage has only unity gain at the first stage, thus improving CMRR. Typical three op amp structure of instrumentation amplifier < img src=' ' alt='Figure 2'> įigure 2. They combine the features of low input bias current, low offset drift and low noise. LTC6081 and LTC6082, however, are continuous time CMOS operational amplifiers, which use a patented methodology to improve their offset voltage, offset voltage drift and CMRR. Chopper stabilized amplifiers, also known as zero drift amplifiers, can achieve superior offset and offset drift by means of offset cancellation, but have clock noise and fold-back noise due to sampling. CMOS amplifiers usually offer inferior offset drift, CMRR, and PSRR specifications and therefore are not suitable for precision applications. The 10-lead DFN package of the LTC6081 offers a shutdown function to reduce each amplifier’s supply current to 2♚.īipolar amplifiers can have low offset and low offset drift, but their nA level input bias current make them inappropriate for high input impedance applications such as photodiode amplifiers. The LTC6081 and LTC6082 have a gain bandwidth product of 3.6MHz, with each amplifier only consuming about 330♚ current for a supply voltage of 2.7 to 5.5V. Their 0.8µV/☌ maximum offset drift, 1pA input bias current,1.3µV p-p of 0.1Hz to 10Hz noise, 120dB open loop gain and 110dB CMRR and PSRR make them perfect for precision applications. The LTC6081 and LTC6082 are dual and quad low offset, low drift, low noise CMOS operational amplifiers with rail-to-rail input and output stages. CMOS Op Amp Outperforms Bipolar Amps in Precision Applications