oscarcot
Junior Member level 1
Hello,
I'm designing a Fully-Differential low noise amplifier for neural signals like the one on the next links:
**broken link removed**
https://www.uni-ulm.de/en/in/instit...or-multi-channel-neural-signal-recorders.html
The amplifiers generate the gain through the capacitor ratio and use a pseudo-resistor (PR) to act as a high-pass filter. As you can see, the input nodes of the amplifier (+) and (-) are between a capacitive voltage divider, so they are kind of floating. Theoretically, the feedback caps C2 should be discharged by the pseudoresistor, bringing the (+) and (-) nodes to the output DC potential. Nevertheless, the pseudoresistor has a very high resistance, usually in Teraohm range, so maybe spectre considers it directly as non-conducting for the DC-analysis.
My problem is: for my layout, the analog-extracted simulation takes the (+) and (-) potentials to 2V, which is already out of the input range of my amplifier. I would like to be able to take this potential to 1V or 1.5V, so my amplifier can work.
To localize the source of the problem, I did the following:
1. I took away all the elements of the circuit, (LNA, CMFB, current sources) and defined the outputs at 1.5V. At the end, only the capacitive voltage divider was there. Vin= 0V, Vout=1.5 and the result was: V(+) = V(-) = 2 V!!, which makes no sense for me. Anyway, this means that the problem was not being caused by my amplifier layout or even the PR.
2. I put all the blocks back, but took out the input caps C1 (C1,C3 for second link) and simulated them as ideal. The V(+) and V(-) were defined as 1.5, which was in the correct input range. I guess this means, only the parasitics of the input caps are causing the problem.
3. I changed the MIM input caps to Poly on Difussion caps, which require substrate contacts connected to GND. Now the voltage V(+) and V(-) go down to 0.3 V, which is also out of range. This helps me think, that at least something can influence this potential.
I tried to make a voltage divider with diodes, hoping to force the simulator to define the voltage at 1.5V. I also connected small resistors which have substrate contacts, but none of these affect the DC solution. I reduced the gmindc and gmin value to 0.01 pS, which helps, but still doesn't bring the potential to the right range.
Does anybody have an idea?
Thanks in advance.
Oscar
I'm designing a Fully-Differential low noise amplifier for neural signals like the one on the next links:
**broken link removed**
https://www.uni-ulm.de/en/in/instit...or-multi-channel-neural-signal-recorders.html
The amplifiers generate the gain through the capacitor ratio and use a pseudo-resistor (PR) to act as a high-pass filter. As you can see, the input nodes of the amplifier (+) and (-) are between a capacitive voltage divider, so they are kind of floating. Theoretically, the feedback caps C2 should be discharged by the pseudoresistor, bringing the (+) and (-) nodes to the output DC potential. Nevertheless, the pseudoresistor has a very high resistance, usually in Teraohm range, so maybe spectre considers it directly as non-conducting for the DC-analysis.
My problem is: for my layout, the analog-extracted simulation takes the (+) and (-) potentials to 2V, which is already out of the input range of my amplifier. I would like to be able to take this potential to 1V or 1.5V, so my amplifier can work.
To localize the source of the problem, I did the following:
1. I took away all the elements of the circuit, (LNA, CMFB, current sources) and defined the outputs at 1.5V. At the end, only the capacitive voltage divider was there. Vin= 0V, Vout=1.5 and the result was: V(+) = V(-) = 2 V!!, which makes no sense for me. Anyway, this means that the problem was not being caused by my amplifier layout or even the PR.
2. I put all the blocks back, but took out the input caps C1 (C1,C3 for second link) and simulated them as ideal. The V(+) and V(-) were defined as 1.5, which was in the correct input range. I guess this means, only the parasitics of the input caps are causing the problem.
3. I changed the MIM input caps to Poly on Difussion caps, which require substrate contacts connected to GND. Now the voltage V(+) and V(-) go down to 0.3 V, which is also out of range. This helps me think, that at least something can influence this potential.
I tried to make a voltage divider with diodes, hoping to force the simulator to define the voltage at 1.5V. I also connected small resistors which have substrate contacts, but none of these affect the DC solution. I reduced the gmindc and gmin value to 0.01 pS, which helps, but still doesn't bring the potential to the right range.
Does anybody have an idea?
Thanks in advance.
Oscar