A center-tapped transformer, with the center tap grounded, is one solution. The other is resistors to GND from each input. Just keep in mind that 100k would give you a nominal 1V common-mode offset, and will load your inputs, which won't happen with the center tapped version.To preserve termination options, no internal
circuitry connects to the input bases of this differential
stage. For this reason, the user must provide DC paths for
the input base currents from a signal source, either through
a grounded termination resistor or by a direct connection
to ground.
Why would a blocking capacitor cause attenuation? Did you provide a path for bias current?dknguyen said:Okay, thanks. New question now...I am thinking cascading two of these VCAs for a max gain of ~80dB (I don't really know how much I Need, but that's what the maximum adjustable gain is between the two), except that each amp has a maximum offset of 8mV. Now this 8mV after it comes out of the first amp is going to get amplified right along with everything else, but tHe simulation shows that if I place a DC block capacitor between the two amps then the net output ends up being attenuating rather than amplifying.
Is there a way to get around this? It seems I need a way to AC couple the two amps without messing around with impedances.
Roff said:Why would a blocking capacitor cause attenuation? Did you provide a path for bias current?
Did you read both of my most recent posts?dknguyen said:Is that what it is? Right now the cap just directly connects the output of one op-amp to the non-inverting input of the other op-amp (the inverting input is grounded). So you are saying that a large resistor to ground between the capacitor and input of the next amp would solve the problem? It has happened with every other op-amp circuit I simulated so far so perhaps that is it.
dknguyen said:Yeah, I thought I remember people just sticking a capacitor on the non-inverting terminal of op amps in a non-inverting amplifier and having it work just fine. I'm not sure why it's not working here. There is a path for the current to ground isn't there since it's in between op-amps and the output of an op-amp has a path to ground?
THis is the circuit currently (except without those two resistors on either side of the DC block capacitor, I added those in as I was messing around with it).
That's the problem. It's a DC blocking capacitor. No DC current can get through it.it's in between op-amps
Yes, you need the resistor for all op amps. Most op amps have much lower bias current than the VCA810 (which is not an op amp). The bias current in CMOS amps is typically picoamps, but you still need a path for DC current. The resistor for a CMOS amp can be megohms.dknguyen said:Here is the whole circuit right now. The output to this circuit goes through that AC block capacitor all the way onto the right into a MAX913 comparator. So if the comparator also has no connections to ground then I also need to provide a resistor for the DC bias currents then again?
Clarification- do I need to add that resistor when AC coupling two op-amps/comparators together every singe time? Or is it done internally on some of them?
If your sim started out with zero charge on the cap, it could run for a long time without showing a problem.dknguyen said:I'm slightly confused now because I ran two simulations for cascaded AC coupled non-inverting amps and cascaded AC coupled buffers both with and without those resistors on the AC coupled input of the second amp and it says it works? I ran them using an actual op-amp, an OPA743. So I was about to come to the conclusion that it was dependent on the op-amp.
It isn't. I'm actually using the transformer to isolate the silicon from the high-voltage bias for the transducer on the primary side of the transformer. My theory is, that any failures are high-voltage DC failures rather than AC ones so that if something fails the transformer should be able to block the high-voltage (otherwise I would need more isolators on the output side of the analog receiver logic- which is what this circuit is).marksanders said:Firstly a differential system using a transformer provides good common mode isolation and unless the cost or size/weight is restrictive then keep it as part of the design. The overall common mode rejection ratio CMRR that is the amount of rejection of common mode signal that is rejected compared to the differential that is required should be high as the common mode impedance from either input transformer connection to 0V is high, lots of Megohms when compared to the differential impedance of the transformer input winding - that is mathematically related to the secondary impedance and turns ratio.
THat capacitor is there as an impedance matching thing (the transducer is capacitive and supposedly has a capacitance of 400-500pF. It never occured to me it might charge up. I'll add something (I guess I shouldn't rely on the amp's differential 1M input impedance huh?). BUt won't the capacitor discharge itself through the transformer winding (maybe LC oscillating as it does so?)marksanders said:Secondly you will want to insure that the opamp works properly. This is normally done by providing a path for the amplifier input biasing currents to the 0V line. Therefore you will need to provide some 100k resistors from the + and - pins to 0V for the amplifier to work properly. If these resistors are not added then the capacitor across the transformer output will slowly charge up to either the + or - rail and damage the amplifier inputs.
marksanders said:Thirdly you will be able to measure some ac voltages on either side of the output transformer referred to 0V. Looking on an oscilloscope and assuming a sinusoidal input then one transformer side will be positive going as the other side is negative going with respect to 0V. There will be voltage gain or loss according to the turns ratio. If you usea signal generator a frequency sweep would confirm the bandwidth of your system.
What you really need to do here is determine is the maximium likely voltage swing to the amplifier + and - inputs - this could damage the amplifier inputs.
If the differential voltage is then outside the amplifier differential spec then you will have to provide voltage excess protection systems using diodes to the supply rails or another voltage reference rail, to 0V or also fitted across the + and - input pins - perhaps back to backdiodes. Opamp data sheets may show some useful varieties. ( Some opamps may need different circuits )
I hope that this approach is at least logical and straight forward
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