If you want to use the TTL signal which you have conditioned from the opto sensor into an analogue intput you will have to create a Frequency To Voltage Convertor on your conditioning cicruit. From memory a Diode and a capacitor / resistor network should suffice. The Diode prevents the capacitor being discharged back into the digital conditioning circuit. This then causes the capacitor to store an average value of frequency. Again from memory any old diode will suffice (1N4005) and a 1uF capacitor together with around a 100K resistor (you may have to tinker with the values a bit). If you don't put a resistor accross the capacitor; then the the capacitor discharges quite slowly into the analogue input due to its very high impedance. Also its discharge rate will change significantly if you try to measure the output with another / different instrument (volt meter). There are other more complex circuits, I am sure every one has thier favorites. Without a conditioning circuit, all you will see is an alias sampling of the square wave input which will be based on the sample interval of the analoge input channel.
I think that the 6025E has a counter input, would not this be the better option to measure a frequency? It does of course depend on the overall configuration that you are aiming for. You need to consider this and the accuracy that you need to achieve. Also you will have noise on the analogue channel that will make it fun. So then you have to decide how you are going to handle the measurement on the analogue channel.
