I-to-V Amp with Offset
OPITOV_OFF.CIR Download the SPICE file
A common application of the I-to-V Amplifier is converting the current output of a DAC to a voltage. The DAC typically outputs a uni-polar current, say 0 to 2 mA. However, often you need to create a bipolar output of say +/-10V. How is this accomplished? A simple modification gets you there - simply add an offset to the output.
TRANS-IMPEDANCE AMP WITH OFFSET
This amplifier acts similar to the summing amp. The currents from both Is and R1 flow through RF creating a total current of Itot = Is + IR1. Because the op amp operation holds node 1 at 0V, the current flows through RF creating a negative output voltage
Substituting the current through R1 as IR1 = V1 / R1, we get
Rewriting slightly we arrive at
What's nice about this equation? You can easily set gain from Is and V1 to create any signal level and offset level you need.
Suppose you have a 12-bit DAC (Digital to Analog Converter) that generates a uni-polar output of 0 to -1 mA for digital words 0 to 4095. However, the application calls for a bipolar output of -5 to +5 V. To achieve this, set RF=10k. The output will swing 0 to +10V for the input current of 0 to -1 mA multiplied by -RF. However, we need to shift this swing down by -5V to get the desired -5 to +5 V. The offset of -5V is accomplished by V1=5V multiplied by -RF/R1 = -1.
CIRCUIT ANALYSIS Current source IS simulates a DAC output of 0 to -1 mA over 10ms. Run a simulation of OPITOV_OFFSET.CIR and plot the input current I(IS). In another window, plot the output voltage V(2). Does V(2) swing -5V to +5V as expected?
HANDS-ON DESIGN Suppose you needed to double the output voltage swing to +/-10V? This is the voltage range used by many motor control systems. Simply double RF from 10k to 20k. Run a simulation and check out V(2). Does it swing a total of 20V as desired. But what about the offset? You may need to adjust R1. For a 20V swing, you need to offset the output by -10V. With V1=5V and RF=20k, what value of R1 achieves the -10V offset? Change R1 and check the output swing.
WARNING - INSTABILITY AHEAD
The implementation of the trans-impedance amplifier is fraught with danger. Even a small stray capacitance can make the output overshoot and ring. Check out the topic on trans-impedance amp stability.
Download the file or copy this netlist into a text file with the *.cir extention.
OPITOV_OFFSET.CIR - CURRENT-TO-VOLTAGE CONVERTER (TRANSIMPEDANCE AMP) * * INPUT CURRENT IS 0 1 AC 1 PWL(0US 0MA 100US -1MA) * * OFFSET VOLTAGE V1 10 0 DC 5VDC R1 10 1 10K * * TRANSIMPEDANCE AMPLIFIER RF 1 2 10K XOP1 0 1 2 OPAMP1 * * OPAMP MACRO MODEL, SINGLE-POLE * connections: non-inverting input * | inverting input * | | output * | | | .SUBCKT OPAMP1 1 2 6 * INPUT IMPEDANCE RIN 1 2 10MEG * gain bandwidth product = DCGAIN x POLE1 = 10MHZ * DCGAIN=100K AND POLE1=100HZ EGAIN 3 0 1 2 100K R1 3 4 1K C1 4 0 1.5915UF * OUTPUT BUFFER AND RESISTANCE EBUFFER 5 0 4 0 1 ROUT 5 6 10 .ENDS * * ANALYSIS .TRAN 0.2US 100US .PROBE .END
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