Other Parts Discussed in Thread: OPA189
Hi all,
Recently, I need a VCCS circuit to charge a capacitor as a time counter. The detailed circuit is shown below.
The VCCS will charge the capacitor and reset when the voltage across the capacitor reaches the threshold voltage.
I found that the OPA 615 contains an OTA which may serve the function. However, I am concern about the peripheral circuit for the OTA.
I know that the input should be base(B) and Emitter(E), and the output should be collector(C). Therefore, there are two possible connections for the VCCS, as shown below.
The first one is that the positive input connects to the signal after subtraction and the negative input connects to the ground. However, in this method, the emitter will be grounded, which is not recommended in the datasheet( PP.13 first paragraph).
The second one is that the positive input connects to the Vc1 and the negative input connects to Vc2, which directly uses the differential signal. Nevertheless, in this method, the emitter may have a large current sinking/sourcing, which may destroy the Vc2 signal, generated from OPA189. I'm not sure is this right or not. In my understanding, the emitter will sourcing/sinking the same amount of current as of the collector. I'm afraid there is a loading effect if the Vc2 is generated from an OPA.
Does anyone have a good idea of how to connect the OTA to enable VCCS function?
By the way, the voltage level of VC1 and VC2 are hundreds of or tens of mV.
