Design Ideas: March 14, 1996
The circuit in Figure 1 addresses applications in which you need to convert a differential-current output to a single-ended voltage. Several analog modules, such as the MC1495 multiplier and the LM1596 modulator/demodulator, provide differential-current outputs. You can adjust R5 to adjust the voltage gain over the range of 5 to 201 to accommodate your application. The LM7121 op amp has a 2.5-GHz gain-bandwidth product at a gain of 190. Because the LM7121 has unlimited capacitive driving capability, you need not provide buffers for load isolation.With >13-MHz bandwidth at a gain of 190 and 25V p-p output-swing capability (35 MHz and 4V p-p with a gain of 5), you can use the circuit for baseband video and audio applications. The output slew rate is >700Vµsec, which allows for a full-power bandwidth in excess of 8 MHz. You can trim the offset with R14; the input-offset temperature coefficient is 30 µV/°C over the -40 to +85°C industrial temperature range. You can adjust the R3 loop-gain trimmer to set the step response (that is, the damping factor) of the amplifier for different gain settings. (DI #1842)