impedanceoperational-amplifieroutputreceiverresistance
Let's say I have an INA134 (http://www.ti.com/product/ina134) from Texas Instruments in my design. I want to take the output of that IC and feed it into a resistor network which can have a flexible input resistance (from 100 ohm to 100k ohm).
What is the reason that the output impedance of the IC is not specified in the datasheet? It's based on a simple opamp circuit after all.
Does it make sense to follow the INA134 by a OPA134 to allow for more drive capability?
The output impedance is, by definition, the impedance seen at the output terminals of the AC model. Thus you only have to apply the definition of impedance: apply an independent voltage source at the output (with arbitrary voltage \$V_s\$) and determine, using usual circuit analysis techniques, the current \$I_s\$ that the voltage source delivers into the terminals (which will depend on \$V_s\$ in a linear fashion, since the AC model is linear).
Afterwards compute the ratio \$\frac{V_s}{I_s}\$ and that is your \$Z_{out}\$:
\$Z_{out} = \dfrac{V_s}{I_s}\$
If you did your computations correctly the \$V_s\$ dependency of the numerator and denominator should cancel out, giving an expression involving only the AC circuits parameters.
It's not clear exactly what you need, but a few things seem clear. The first is that your 74S124 produces a digital output, so you can use a digital buffer rather than an op amp. Furthermore, your -1.5 volt output occurs as a result of bad physical layout interacting with the sharp fall time of the VCO, and does not represent a guaranteed effect.
Buffering the VCO with a high-speed buffer such as the 74FCT244 would seem to be your best bet. These are available from sources such as Digikey for a buck apiece. The circuit would look like
simulate this circuit – Schematic created using CircuitLab This will not provide perfect amplitude stability, but should probably be adequate for your needs. The output current capacity of the FCT244 is great enough that you should not have problems.
Best Answer
The output drive capability is defined in a few places in the datasheet, but not guaranteed except at 2K load. The below graph is based on +/-18V supplies.
As you can see you can get 20mA easily out of it, with 15V swing. With a 100R load, the output capability would be a couple of volts or so safely.
I don't see why you'd want to follow this part with another such as the OPA134 that has similar or less current capability.
Keep in mind that the closed-loop output impedance on both amplifiers will appear very low, up until the point where you hit the current limit (or when thermal protection kicks in).