Hi
on the QA472 documentation webpage for Preamp 2, the max. DC input voltage is specified as -1V to +50V. In the datasheet the max. DC input voltage is specified as +/- 50V. I hope the datasheet specification is correct.
BR
Andy
Hi @Avo, thanks for pointing this out! The JFET Pre (PRE2) is limited to -1V to 50V, with a requirement that current be limited to <100mA for 50mS and 20 mA continuous. I have updated the datasheet on the QA472 product page.
Hello Matt
thank you for the quick clarification. I was measuring the power supply of my latest preamp design with the QA472 to amplify the noise. It is only +/- 5V and nothing was damaged but in the future I better use external, non polarized capacitors. I was expecting that the QA472 has non polarized input capacitors, the same as on the QA40x inputs. Maybe it would be good if you could provide a (simplified) schematic of the input section. I remember discussions in the forum about the QA40x where the input stage schematic would have been useful.
Thank you and best regards
Andy
Hi @AVO, the front-end is very close to the front-end from the JFE150 data sheet. Note that TI also has some input protection steering diodes that aren’t shown in the image below, but there’s more detail in the app notes. I think the Rg could be increased to help with current limiting, but the aim of the circuit is the lowest noise possible, and there’s a little but not a lot of room to increase it before you start hurting overall noise.
Remember, just putting a capacitor in series doesn’t solve the problem. The cap blocks DC over time, but when you first connect a series cap to a 20V supply, there’s a large swing on the other side of the cap upon connection (run it in spice to see). An that is where the series R is needed to limit the flow.
Very low noise amps will always need some TLC because you are trying to optimize for many things. it’s a challenge, and I think TI’s integrated steering diodes on the JFE150 are a great step forward.
