High speed opamp amplifier with LT1222 - Page 1
Forum section: electronics / projects

author: Dajgoro
author_rank: Frequent Contributor
timestamp: December 12, 2013, 12:11:06 am
content: I recently got a LT1222 opamp, and I built a small test circuit with it.I built the circuit that I found in the datasheet, but without the diodes on pin 5.I also added a 100 ohm resistor to the ground at the output, and the circuit is build tightly(point to point) above a ground plane pcb.http://cds.linear.com/docs/en/datasheet/1222fc.pdfAt the output I got a rise time of 400ns, and a fall time of 4us, which makes it run at max 100kHz. I need to be able to amplify up to 50MHz with a gain of 10.What is pin 5 (Comp)? What does it do?How do I make it go up to 50MHz?

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: December 12, 2013, 12:34:30 am
content: The compensation pin is covered on page 7, along with its affect on bandwidth. I think that answers your other questions as well.

author: prenato
author_rank: Regular Contributor
timestamp: December 12, 2013, 01:43:04 am
content: Some other suggestions would be to check the slew rate specs of the opamp versus what you need in your application, and also what load capacitance you have in the output?You mentioned you need to amplify "up to 50 MHz", but didn't say if the input is sinusoidal or a square wave, or something else? (A square wave at 50MHz takes much more than 50MHz bandwidth of course; more like 250 MHz to get the 5th harmonic through).

author: Marco
author_rank: Super Contributor
timestamp: December 12, 2013, 02:16:28 am
content: What's the amplitude of your input pulse and your power supply voltages?

author: Dajgoro
author_rank: Frequent Contributor
timestamp: December 12, 2013, 07:08:26 pm
content: The input signal is a sine, so it doesn't have higher frequency components.I have 5V and 12V in my circuit, and what I am trying to do is to amplify the output of the AD9850 DDS, so that I can have a decent signal output.

author: akis
author_rank: Frequent Contributor
timestamp: December 12, 2013, 09:48:13 pm
content: Thinking aloud: Maybe you need a better layout at those frequencies? Is the ground plane underneath the op-amp grounded? Have you maybe connected a probe which also inadvertently grounds the circuit?

author: Dajgoro
author_rank: Frequent Contributor
timestamp: December 12, 2013, 09:51:07 pm
content: Quote from: akis on December 12, 2013, 09:48:13 pmYes, it is all grounded, and I even tried soldering a bnc connector directly on the board, but that didn't change much. If I change the amplification factor, it kinda starts working, but then it starts oscillating.

author: Unknown
author_rank: Unknown
timestamp: December 12, 2013, 11:04:32 pm
content: The AD9850 output is getting pretty ratty by 50MHz; a more practical limit would be something like 30 or 40MHz depending on how much distortion you're willing to accept.Why did you choose the LT1222 over the LT1253? The latter seems to be more commonly used with the AD9850 (e.g.), and seems to have a much higher GBW than the LT1222.Disclaimer: I haven't tried the LT1253 myself; I've got some lined up for my next order.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: December 12, 2013, 11:16:22 pm
content: Quote from: Tac Eht Xilef on December 12, 2013, 11:04:32 pmYes, I know. 40MHz is the most that I can get out of it, but I wouldn't like for the amplifier to struggle at that frequency.Quote from: Tac Eht Xilef on December 12, 2013, 11:04:32 pmBecause I know nothing about high speed opamps, and I happened to find the LT1222 by chance. I got it from ebay, and I had to wait quite a bit for it to arrive.

author: Unknown
author_rank: Unknown
timestamp: December 13, 2013, 12:04:04 am
content: Quote from: Dajgoro on December 12, 2013, 11:16:22 pmFair enough. It was just that I've been gathering ideas for a suitable agc / amp / line driver myself and I don't think I've ever seen the LT1222 suggested anywhere, while the LT1253 or the similar Maxim chips are common suggestions (and cheaper). Thought there might have been something I was overlookingedit: I said earlierQuote from: Tac Eht Xilef on December 12, 2013, 11:04:32 pmScratch that; for some reason I thought it was 900 vs 500. It's actually only 90 (albeit flat to 30MHz). I might look at the LT1395 & co; they're supposed to be flat to 50~100MHz depending on gain & load impedance.