Let’s add them in parallel with the input resistors, as in Figure 3. They represent the input common-mode capacitance on the non-inverting/inverting inputs and the differential input capacitance. Let’s add it as in figure 2.Īs in the case with the input resistance, the input capacitance of an op amp can be specified as three input capacitors, Cin1, Cin2 and Cin. Because of that we will estimate a 20 ohm series resistor with the op amp output. The output resistance is not specified as well for this op amp. However, other op amps, built with different technologies, will have lower input resistance values and they will make a difference in the SPICE model. Having these resistors in the model will not make too much of a difference, therefore the model will work very well with or without them for this particular op amp. Let’s estimate the common-mode resistance as 10 12 ohms and the differential input resistance as 1 Gohm. As such, the input resistance is very high, up to 10 12 ohms. This is most likely due to the fact that ADA4004 is made with the Analog Devices’ new iPolar technology which is an enhanced JFET technology. The ADA4004 does not have a specification for the input resistance. As such, in our Multisim model let’s add these resistors in the VCVS input as in Figure 2. These resistors represent the input common-mode resistance on the non-inverting/inverting inputs and the differential input resistance. The input resistance can be defined in the datasheet of an op amp as a group of three resistors: Rin1, Rin2 and Rin. Check this link for info where to find Multisim, and also other simulation programs.) It was developed by Electronics Workbench and now owned by National Instruments. ( Note: Multisim is a simulation program with a SPICE engine. Let’s build this op amp model in Multisim (see Figure 1). This means a gain of 500000 at DC level, so my VCVS will have the transfer function of 500000 V/V. The datasheet shows that the open-loop gain, A VO, is minimum 500V/mV. At the time this article was published, Analog Devices did not release a SPICE model for this op amp. Let’s build a model for the Analog Devices’ ADA4004, which is a precision operational amplifier. This VCVS also helps in setting the Op Amp open-loop gain. The simplest Op Amp model is a Voltage-Controlled-Voltage-Source (VCVS) (see my article :An Ideal Operational Amplifier Simulation Model).
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