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AMP04FS Datasheet(PDF) 8 Page - Analog Devices |
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AMP04FS Datasheet(HTML) 8 Page - Analog Devices |
8 / 16 page AMP04 REV. A –8– Compensating for Input and Output Errors To achieve optimal performance, the user needs to take into account a number of error sources found in instrumentation amplifiers. These consist primarily of input and output offset voltages and leakage currents. The input and output offset voltages are independent from one another, and must be considered separately. The input offset component will of course be directly multiplied by the gain of the amplifier, in contrast to the output offset voltage that is in- dependent of gain. Therefore, the output error is the dominant factor at low gains, and the input error grows to become the greater problem as gain is increased. The overall equation for offset voltage error referred to the output (RTO) is: VOS (RTO) = (VIOS × G) + VOOS where VIOS is the input offset voltage and VOOS the output offset voltage, and G is the programmed amplifier gain. The change in these error voltages with temperature must also be taken into account. The specification TCVOS, referred to the output, is a combination of the input and output drift specifica- tions. Again, the gain influences the input error but not the out- put, and the equation is: TCVOS (RTO) = (TCVIOS × G) + TCV OOS In some applications the user may wish to define the error con- tribution as referred to the input, and treat it as an input error. The relationship is: TCVOS (RTI) = TCVIOS + (TCVOOS / G) The bias and offset currents of the input transistors also have an impact on the overall accuracy of the input signal. The input leakage, or bias currents of both inputs will generate an addi- tional offset voltage when flowing through the signal source re- sistance. Changes in this error component due to variations with signal voltage and temperature can be minimized if both input source resistances are equal, reducing the error to a common- mode voltage which can be rejected. The difference in bias cur- rent between the inputs, the offset current, generates a differen- tial error voltage across the source resistance that should be taken into account in the user’s design. In applications utilizing floating sources such as thermocouples, transformers, and some photo detectors, the user must take care to provide some current path between the high impedance in- puts and analog ground. The input bias currents of the AMP04, although extremely low, will charge the stray capacitance found in nearby circuit traces, cables, etc., and cause the input to drift erratically or to saturate unless given a bleed path to the analog common. Again, the use of equal resistance values will create a common input error voltage that is rejected by the amplifier. Reference Input The VREF input is used to set the system ground. For dual sup- ply operation it can be connected to ground to give zero volts out with zero volts differential input. In single supply systems it could be connected either to the negative supply or to a pseudo- ground between the supplies. In any case, the REF input must be driven with low impedance. Noise Filtering Unlike most previous instrumentation amplifiers, the output stage’s inverting input (Pin 8) is accessible. By placing a capaci- tor across the AMP04’s feedback path (Figure 6, Pins 6 and 8) 2 3 8 1 6 5 IN(–) IN(+) INPUT BUFFERS R GAIN 100k REF 100k VOUT 11k 11k R GAIN C EXT ƒ LP = 1 2 π (100k) C EXT Figure 6. Noise Band Limiting a single-pole low-pass filter is produced. The cutoff frequency (fLP) follows the relationship: f LP = 1 2 π (100 kΩ) C EXT Filtering can be applied to reduce wide band noise. Figure 7a shows a 10 Hz low-pass filter, gain of 1000 for the AMP04. Fig- ures 7b and 7c illustrate the effect of filtering on noise. The photo in Figure 7b shows the output noise before filtering. By adding a 0.15 µF capacitor, the noise is reduced by about a factor of 4 as shown in Figure 7c. 7 1 6 5 4 3 2 8 +15V –15V 100 0.15 µF Figure 7a. 10 Hz Low-Pass Filter 10 90 100 0% 5mV 10ms Figure 7b. Unfiltered AMP04 Output |
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