SECTION 1 SINGLE-SUPPLY AMPLIFIERS - Analog Devices

the circuit has an adjustment range of ±20%. Note that sufficient headroom existsin the gain trim to allow at least a 10% overrange (+4.4V).AD626 HIGH-SIDE CURRENT MONITOR INTERFACEFigure 1.17INSTRUMENTATION AMPLIFIER INPUT-STAGERECTIFICATIONA well-known phenomenon in analog integrated circuits is RF rectification,particularly in instrumentation amplifiers and operational amplifiers. Whileamplifying very small signals, these devices can rectify unwanted high-frequency,out-of-band signals. The results are DC errors at the output in addition to thewanted sensor signal. Unwanted out-of-band signals enter sensitive circuitsthrough the circuit's conductors which provide a direct path for interference tocouple into a circuit. These conductors pick up noise through capacitive, inductive,or radiation coupling. Regardless of the type of interference, the unwanted signal isa voltage which appears in series with the inputs.All instrumentation and operational amplifier input stages are either emittercoupled(BJT) or source-coupled (FET) differential pairs with resistive or currentsourceloading. Depending on the quiescent current level in the devices and thefrequency of the interference, these differential pairs can behave as high-frequencydetectors. As it has been shown in [1], this detection process produces spectralcomponents at the harmonics of the interference as well at DC. It is the DCcomponent that shifts internal bias levels of the input stages causing errors, whichcan lead to system inaccuracies. For a complete treatment of this issue, includinganalytical and empirical results, the interested reader should consult Reference [1].Since it is required to prevent unwanted signals and noise from entering the inputstages, input filtering techniques are used for these types of devices. As illustratedin Reference [1], this technique uses an equivalent approach suggested foroperational amplifiers. As shown in Figure 1.18, low-pass filters are used in series29