I have a situation that I've run into a few times. I have a digital output card in my system that uses SSRs. I'm trying to interface to a device (referred to as my DUT from here on) that has TTL digital inputs that are internally pulled up to 5 volts. To active the input, I need to short the input to ground.
However, using the SSR to connect the input to ground results in around 40 mV across the SSR and the DUT doesn't report this as ON.
I don't want to confuse the issue, but the DUT appears to be doing an analog measurement on the pin because there are certain functions on the DUT that are triggered by this pin being pulled low, and those functions do occur when I pull it low. However, there's a STATUS command I can send to the DUT to ask if this pin is low and it reports it as HIGH unless I actually short the input and ground with a piece of wire. This leads me to believe there are a couple different functions that query the status of this pin and that they must have different thresholds.
So my real question: is there an electrical way to minimize the voltage drop across the SSR in this scenario so the input of my DUT sees something closer to 0V?
Edit: I realize this is a hard question to answer without links to specific components, but in this case the SSR is a part of a digital output card (and thus I don't know real details about the specific implementation) and the DUT is a new device that might as well be a black box. I suspect the answer to my original question is "no" but thought I'd check to see if there was some general approach to making an SSR look a little more like a relay contact in certain specific applications.