Hello Every body,
I am generating 3Vp-p from DAC inside PSOC 5LP. It looks very nice as long as i do not connect it with bridge (in order to excite the bridge). When i connect it, the voltage drops from 3 to 1/3 of that. I tried to place a voltage follower at the output of the DAC but as a result my signal becomes very noise and drops too (but not much).
I am failed to find the reason. Can any body have any idea?
I attached the snap shot of my design.
Looking forward for your suggestions.
When the WaveDAC voltage drops when connected to your design it is quite probable that the input resistance of your bridge is too low.
Connecting an amp/follower and getting more noise could be picked-up noise or something like a feed-back? Not quite sure.
Thankyou for your reply.
The input resistance of the bridge is approx 350 ohms. The output voltage of the DAC Max of 3 V. So the current should be around 8.5mA. Even if we put the voltage follower inside the PSOC (the psoc opamp can deliver upto 25mA) we should be safe. But we are not.
Even in this configuration the voltage drops about 2/3.
Looking forward for your suggestion.
In my datasheet for the PSoC5 OpAmp I find 10mA as source or sink max current.
Moreover, the switch resistance internal to the PSoC5 to route the output of the opamp to the pin can be in the magnitude of your bridge resistance (depending on the choosen pin). Check this using the Ohm-meter int the analog viewe of the .cydwr-file.
I measured the internal resistance but it show 750 ohms. If it is due to the internal resistance then it should not happend if i put the voltage follower outside the psoc. But it does.
Make sure in the Wavedac8 configurator, range selection, you select the buffered
parameter version. Also I assume you chose the Wavedac as a voltage output, not a
current output. If latter the output Z much higher.
The OpAmp follower may source/sink 25mA, but not at the level of the input voltage.
Yes, it has a 12 ohm out put Z per the graph I posted. In fact NO OpAmp will follow exactly
the input V due to finite Aol and Output Z.
Be sure to use as output pin the defaulted pin for the amplifier, only this guarantees a Zero Ohm resistance (no switch needed)
The routing path is NOT 0 ohms. Its lower for the assigned pins, but not 0.
Yes, that's right, Dana. Would be nice to have a Zero Ohms part in a PSoC, I would use it instantly for my modell maglev train ;-))
Nonetheless, in analog view the Ohmmeter shows ~0 Ohms.
Oh Yes Bob and Dana, You are great :).
I got the solution :).
Now i am using the proper pin and my voltage is no more dropping any more :).
But can you tell me why i am getting a noisy signal if i attached an external voltage follower?
Thankyou so much Bob & Dana.
The ohmmeter is a crude instrument as you know. Its not a Kelvin capable tool,
just a routing estimator from sysnthesizer/router.
Awais, the noise can be many things. Layout, coupling, ground bounce, poor
How much noise are you experiencing ?
Use your scope on infinite persistence and look at grounds (referred to ground point entering board),
supply rails. Polymer Tants an order of magnitude better than standard tants, F vs Z, for bulk bypassing.
Look at its frequency to see if you can tie it to some timer, clock, counter you
If driving motor loads very suspicious of grounds and unclamped L loads.
2V P-P very large noise. Are you using large C loads, maybe the OpAmp
phase margin disappeared.
Your scope and probe set to the right attenuation when measuring noise ?
I am Driving a simple bridge circuit. Yes my scope and probes are setup at the right attenuation level. But i think you are right that this might be due to grounding.
Any ways, I will be using internal voltage follower for my signal.
Thankyou so much once again for helping me .