GENERALIZED EXTERNAL PROTECTION SCHEME AGAINST INPUT OVERVOLTAGE ABUSE AND OUTPUT VOLTAGE PHASE REVERSAL IN SINGLE-SUPPLY OP AMPS Rs Value for Rs provided by manufacturer or determined empirically a RFB may be required for high bias current devices a D1 and D2 can be Schottky diodes(Check their capacitance and leakage current first Figure 7.8 The external input series resistance, Rs, will be provided by the manufacturer of the amplifier, or determined empirically by the user with the method previously shown in Figure 7.2 and Eq 7 1. More often than not, the value of this resistor will provide enough protection against output voltage phase reversal, as well as limiting the fault current through the Schottky diodes It is evident that whenever resistance is added in series with an amplifiers input its offset and noise performance will be affected. The effects of this series resistance on circuit noise can be calculated using the following equation en, total en op amp +(en,Rs ) +(&, in,op amp) The thermal noise of the resistor the voltage noise due to amplifier noise current flowing through the resistor, and the input noise voltage of the amplifier are added together (in root-sum-square manner, since the noise voltages are uncorrelated) to determine the total input noise and may be compared with the input voltage noise in the absence of the protection resistor A protection resistor in series with an amplifier input will also produce a voltage drop due to the amplifier bias current flowing through it This drop appears as an increase in the circuit offset voltage(and, if the bias current changes with temperature, offset drift). In amplifiers where bias currents are approximately equal, a resistor in series with each input will tend to balance the effect and reduce the error The effects of this additional series resistance on the circuit's overall offset voltage can be calculated1 1 GENERALIZED EXTERNAL PROTECTION SCHEME AGAINST INPUT OVERVOLTAGE ABUSE AND OUTPUT VOLTAGE PHASE REVERSAL IN SINGLE-SUPPLY OP AMPS Figure 7.8 The external input series resistance, Rs , will be provided by the manufacturer of the amplifier, or determined empirically by the user with the method previously shown in Figure 7.2 and Eq. 7.1. More often than not, the value of this resistor will provide enough protection against output voltage phase reversal, as well as limiting the fault current through the Schottky diodes. It is evident that whenever resistance is added in series with an amplifier's input, its offset and noise performance will be affected. The effects of this series resistance on circuit noise can be calculated using the following equation. En,total (en,op amp ) ( ) ( ) 2 + en,Rs 2 + Rs amp 2 = × in,op The thermal noise of the resistor, the voltage noise due to amplifier noise current flowing through the resistor, and the input noise voltage of the amplifier are added together (in root-sum-square manner, since the noise voltages are uncorrelated) to determine the total input noise and may be compared with the input voltage noise in the absence of the protection resistor. A protection resistor in series with an amplifier input will also produce a voltage drop due to the amplifier bias current flowing through it. This drop appears as an increase in the circuit offset voltage (and, if the bias current changes with temperature, offset drift). In amplifiers where bias currents are approximately equal, a resistor in series with each input will tend to balance the effect and reduce the error. The effects of this additional series resistance on the circuit’s overall offset voltage can be calculated: