AnsweredAssumed Answered

30 MHz amplifier to drive isolation transformer

Question asked by vb_ch on May 27, 2013
Latest reply on May 29, 2013 by harryh



I am trying to design a low-noise amplifier with 30 MHz bandwidth that would be able to drive an RF transformer (1:1) to go across an isolation barrier. The source signal is differential and low impedance, low level (mV) with a huge and fast varying common mode (0 to 1kVp in 10-50 us), hence the need for isolation before going to a signal acquisition system.


First I must say I am new to design at these frequencies, I am more familiar with sub-MHz designs... I was thinking about using two ADA4899-1 in non inverting configuration (like the first stage of an instrumentation amplifier, without the difference amplifier 2nd stage) and directly drive the transformer between the outputs of these opamps. I have also considered the ADA4895-1 as an alternative with higher gain. Of course the power supply (dual 5V) is isolated too via a DC/DC converter. I am currently using 10kOhm bias resistors to floating ground on both positive inputs, though it might be too large (?).


However, I am running into a couple of issues. I never managed the ADA4895-1 to be stable (gain 25, Rf=240 Ohm) without adding load capacitors (at least 10nF behind a 24 Ohm series output resistor) and feedback capacitors (typ. 4.7pF). Is that expected behavior ? The bandwidth flatness is also very far from my expectations above 1 Mhz, although the transformer is supposed to be flat (for 50 Ohm:50 Ohm) between 10kHz to 20MHz at least. I am really getting resonances (plus noise peaking) above 4 MHz.


So first, is the chosen design usable, or should I consider something completely different ?


Thanks for any help !