Low noise amplifier with rail splitter - Page 1
Forum section: electronics / projects

author: Mrt12
author_rank: Regular Contributor
timestamp: February 24, 2023, 01:59:08 pm
content: Hi,I want to do some noise measurements. For this, I would like to use an AD797 op-amp with 40dB gain.However, I don't want to use a negative voltage rail. Because it is very easy in my case to have my measurement signal offset by a couple volts, I wonder whether it is possible to use a rail splitter + appropriate filtering to have the AD797 perform similar as if I was using a positive/negative power supply? is that possible?

author: rfclown
author_rank: Frequent Contributor
timestamp: February 24, 2023, 02:13:27 pm
content: I don't know why a rail splitter would help with the signal having an offset voltage. The "negative voltage rail" isn't a reference in a dual supply op-amp. If you have a DC offset into a 40 dB gain amp, you have to deal with that whether you have a single or dual supply op-amp. DC blocking is the easiest method.

author: Mrt12
author_rank: Regular Contributor
timestamp: February 24, 2023, 02:21:43 pm
content: Hmm I probably explained it a bit misleading.Attached is my schematic. The signal coming out of the SRA-1 mixer is offset by VCC/2. I want to amplify the noise coming from the mixer. So I use the AD797 there and I also use the same VCC/2 reference for it.I wonder whether, from a noise viewpoint, this is much worse than having everything referenced to ground and use a negative rail.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2023, 02:51:21 pm
content: There is no real issue to use a virtual ground. Just make sure it is used consequently - in this examplte the filter capacitor C1 should also go to the virtual ground.The noise from creating the virtual ground can be seen as part of the supply noise.

author: iMo
author_rank: Super Contributor
timestamp: February 24, 2023, 03:02:26 pm
content: Those mixers should see 50ohm on all ports over the entire frequency range of interest, afaik..

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 24, 2023, 03:19:44 pm
content: Only if you are interested in freedom from intermodulation.Even then the LO port is not so important. Having enoughpower on the LO port is important. This is seldom a problemwhen making phase noise measurements.I would also choose a +17 dBm mixer. It gives dynamic range.When used as a phase detector, 400 Ohms or so on theIF side gives a way better Kpd. If it feels good, you mayadd a lower termination for RF with a small series capso that it affects only 3 MHz and above.The 47 Ohm series resistor creates about as much thermalnoise voltage as the op amp.The otherwise fine LT1028 op amp has the drawback of anoise mole hill at 300-400 KHz.The ADA4898 works fine, also. The -2 version was reallyattractive against the 797 by providing 2 opamps at theprice of one 797. AD seem to have corrected that. :-(I have a preamp with 10*2 ADA4898 in par, yielding 200-220pV/rt Hz.<https://arxiv.org/abs/physics/0608211>or everything atwww.rubiola.org.or a lot of things from the timefreq group at nist.gov.Their URLs shuffle around, impossible to track.Gerhard

author: Terry Bites
author_rank: Super Contributor
timestamp: February 24, 2023, 09:06:23 pm
content: If Vcc/2 is properly bypassed it will not affect the noise mesurement. (Unless you're an Audiofool).As it stands you have no defined noise measurement bandwith set.This matters if you're going to get meaningful noise measurments.See attached idea for the noise part.The +in on AD797 sees Vcc/2 via R2. The U2 integrator (LPF?) will ramp to rail after switch on.It would not work with a dual supply either, you'd have (1+(R3/R4))(Vos TL072) ramping the circuit into insanity.This needs a rethink. Maybe use an AC gain stage, a simple diode detector and RC filter?Better still, use a log detector ic, eg AD8307 then you'll becomming an RF kinda person.

author: David Hess
author_rank: Super Contributor
timestamp: February 24, 2023, 11:47:27 pm
content: Quote from: Mrt12 on February 24, 2023, 01:59:08 pmIt is certainly possible to make an active rail splitter that provides the same noise performance as a true dual bipolar supply, it is just more difficult because class-AB instead of class-A operation is required and typical integrated class-AB output stages are not designed to drive capacitive loads.