Datasheet AMP04 (Analog Devices) - 9

AMP04 Compensating for Input and Output Errors Noise Filtering
To achieve optimal performance, the user needs to take into Unlike most previous instrumentation amplifiers, the output account a number of error sources found in instrumentation stage’s inverting input (Pin 8) is accessible. By placing a capaci- amplifiers. These consist primarily of input and output offset tor across the AMP04’s feedback path (Figure 6, Pins 6 and 8) voltages and leakage currents. The input and output offset voltages are independent from one
CEXT
another, and must be considered separately. The input offset component will of course be directly multiplied by the gain of
100k
the amplifier, in contrast to the output offset voltage that is
RGAIN IN(–)
independent of gain. Therefore, the output error is the domi-
INPUT BUFFERS VOUT
nant factor at low gains, and the input error grows to become
IN(+)
the greater problem as gain is increased. The overall equation for offset voltage error referred to the output (RTO) is:
11k
VOS (RTO) = (VIOS × G) + VOOS
1 11k LP = 2 (100k ) CEXT
where VIOS is the input offset voltage and VOOS the output offset
100k
voltage, and G is the programmed amplifier gain. The change in these error voltages with temperature must also
REF
be taken into account. The specification TCVOS, referred to the Figure 6. Noise Band Limiting output, is a combination of the input and output drift specifica- a single-pole low-pass filter is produced. The cutoff frequency tions. Again, the gain influences the input error but not the (f output, and the equation is: LP) follows the relationship: TCV OS (RTO) = (TCVIOS × G) + TCVOOS f = 1 LP In some applications the user may wish to define the error con- 2π (100 kΩ) C EXT tribution as referred to the input, and treat it as an input error. The relationship is: Filtering can be applied to reduce wide band noise. Figure 7a shows a 10 Hz low-pass filter, gain of 1000 for the AMP04. TCVOS (RTI) = TCVIOS + (TCVOOS / G) Figures 7b and 7c illustrate the effect of filtering on noise. The The bias and offset currents of the input transistors also have an photo in Figure 7b shows the output noise before filtering. By impact on the overall accuracy of the input signal. The input adding a 0.15 µF capacitor, the noise is reduced by about a leakage, or bias currents of both inputs will generate an addi- factor of 4 as shown in Figure 7c. tional offset voltage when flowing through the signal source resistance. Changes in this error component due to variations
+15V
with signal voltage and temperature can be minimized if both
100k
input source resistances are equal, reducing the error to a
0.15 F
common-mode voltage which can be rejected. The difference in bias current between the inputs, the offset current, generates a differential 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
–15V
care to provide some current path between the high imped- ance inputs and analog ground. The input bias currents of Figure 7a. 10 Hz Low-Pass Filter 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 resis-
5mV 10ms
tance values will create a common input error voltage that is
100 90
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
10 0%
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. Figure 7b. Unfiltered AMP04 Output –8– REV. C