Greetings from a short-time lurker, first-time poster.
I'm wondering if anyone has experience with the LM1972 volume-control chip from NSC. I am trying to use this device with a single 12V supply rail, and have been running into oscillation problems in the unity-gain JFET op-amp buffer that follows it.
The LM1972 data sheet is not very good about providing circuit specifics. They recommend using a JFET-input unity-gain buffer at the LM1972's output to keep from loading the LM1972 and producing clicks when the attenuation level changes. However, the 1972's output impedance changes over a pretty wide range depending on the level it's set to, and it seems that there are always a few spots in the adjustment range where the unity-gain follower will break into oscillation.
I have tried the LF356 as well as NSC's recommended LF412 as the buffer with the same results. I have tried hardwiring the inverting input directly to the output, as well as using a 1K resistor. I believe that supply bypassing and the like are adequate. The oscillation occurs regardless of the load (or lack thereof) at the op-amp's output.
Has anyone here successfully used the LM197x chips? There seem to be no app notes or example circuits anywhere on the web for this chip. What, exactly, does its unity-gain buffer need to look like?
I'm wondering if anyone has experience with the LM1972 volume-control chip from NSC. I am trying to use this device with a single 12V supply rail, and have been running into oscillation problems in the unity-gain JFET op-amp buffer that follows it.
The LM1972 data sheet is not very good about providing circuit specifics. They recommend using a JFET-input unity-gain buffer at the LM1972's output to keep from loading the LM1972 and producing clicks when the attenuation level changes. However, the 1972's output impedance changes over a pretty wide range depending on the level it's set to, and it seems that there are always a few spots in the adjustment range where the unity-gain follower will break into oscillation.
I have tried the LF356 as well as NSC's recommended LF412 as the buffer with the same results. I have tried hardwiring the inverting input directly to the output, as well as using a 1K resistor. I believe that supply bypassing and the like are adequate. The oscillation occurs regardless of the load (or lack thereof) at the op-amp's output.
Has anyone here successfully used the LM197x chips? There seem to be no app notes or example circuits anywhere on the web for this chip. What, exactly, does its unity-gain buffer need to look like?
No, both the LF412 and LF356 are unity-gain stable. The buffer and the PA that comes after it don't exhibit any stability problems until the LM1972 is connected.
Actually, I think I answered my own question a few minutes ago... neither of these opamps are designed for rail-to-rail service, and the LM1972's output DC offset falls below 0.7V right where the oscillation happens. I'm going to have to run these guys off a split supply, it looks like.
Actually, I think I answered my own question a few minutes ago... neither of these opamps are designed for rail-to-rail service, and the LM1972's output DC offset falls below 0.7V right where the oscillation happens. I'm going to have to run these guys off a split supply, it looks like.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.