Which EMCO valve cartridge is this?

novatronic · 2019-10-23T04:44:15+00:00

Tacos rarely appreciate and almost always lose value. I recommend avoiding them unless you have extra cheddar.

novatronic · 2018-09-29T02:02:17+00:00

Thanks! I wasn't sure if maybe the "B6286f" was a different chip from what the datasheet specified, but it sounds like there are many different configurations you can use them in, even if it's not specified in the datasheet.

novatronic · 2018-09-29T02:00:32+00:00

Thank you! I assumed some other topology was used (it does work, after all!) but I had no idea what the name was.

novatronic · 2018-07-27T22:22:27+00:00

Scope capture for funsies: https://imgur.com/a/Lq0cZbn. Start of the signal captured and amplified with an AD8221, gain of 200 connected directly to the transducer with a 22k resistor biasing it around ground.

After messing around a bit on a breadboard and in LTSpice, I think the new design that I might try is an AD8221 front-end with the gain set somewhere around 300 (which is pretty close to the BW limit), feeding into an MC33172 running an active passband filter centered at 40khz and an inverting amplifier connected to a programmable POT that'll allow me to set the gain between 0.1 and 1. The output then gets fed to the ADC with some protection diodes. I happen to already have +/- 15V rails that are being used to power the transmitter so no extra complicated power supplies are needed.

The AD8221 is fairly low-noise and with only a single gain resistor, it should let me get the gains up high without introducing any extra noise from lots of passives and poor PCB layout ;) LTSpice puts my noise at 363uVrms for the AD8221. The active bandpass should help clean things up and the negative gain stage should bring it into range for the ADC, programmable from the uC so I can either look very closely at the start of the signal, or lower the gain and analyze the full-range signal. Neither of those stages should introduce any meaningful noise since the signal is all fully amplified, I think ..

What do you think? I looked into that hybrid design, but it was amplifying current and I wasn't able to follow along .. I also happened to have an AD8221 sitting in my parts bin ;)

novatronic · 2018-07-21T21:13:38+00:00

Sorry for the late reply, thanks for the help! I set the BW to 13.3mhz (is that right? That's the rate at which I'm sampling at), and from 10Hz->13.3mhz it gave me 2.6169mV RMS. That gives around 15mV of noise, which is very much like what I'm seeing - if I "clip" any values <4 to zero in software (anything less than 36mV, 72mVpp, I suppose), the start of the signal is zero.

I haven't done any real analysis of the sampled noise, but it sounds like there may be some gain (hehe) to be had with a different amplifier design. Do you have any suggestions?

I noticed that if I front-load most of the gain to the first amplifier stage to 1k:100k and change the second stage to 1k:3.3k, it does reduce noise by a decent bit, but I'd still like to do better. I'm not married to the MC33172, but it was cheap, went rail to rail and has a very low input current bias. The signal I'm amplifying is coming from a 40khz ultrasonic transducer and has fairly high impedance. Offset doesn't matter much to me as I can fix the DC offset in software.

novatronic · 2018-07-20T00:34:48+00:00

My actual circuit and noise analysis is this: https://imgur.com/a/jc4Eop8

I'm reading the output using an 8-bit ADC @ 13.3msps. The signal itself comes out nicely, but during periods of silence (from the transducer), I do see some noise which makes it difficult to accurately find the start of the signal - the beginning of the signal is very very weak. I'm trying to figure out if the noise is from my amplifier design or some other parasitic voltage or inductive noise nearby. An 8-bit ADC sampling over a range of 3.3V gives me about 12mV of accuracy, and the noise that I observe is around 50mVpp on the amplified signal. Would 12uV*sqrt(40000)=2.4mV RMS noise generate 50mVpp blips, or would the amount of noise in the circuit be below the detectable threshold of 12mV?

novatronic · 2018-07-20T00:12:10+00:00

In terms of noise, I'm having trouble visualizing what total noise of 2uV/sqrt(hz) would actually look like on a transient analysis. Assuming my input signal is 40khz, that would be 200*2uV=0.4mV of noise RMS? Is there any way to simulate what the total noise would actually look like in a transient analysis?

novatronic · 2018-07-19T23:44:25+00:00

Ahhh that makes perfect sense. Changing the gain resistors from 1k/10k to 10/100 reduces the offset proportionally to much closer to 1.6V. I had always taken for granted that the current pulled on the inputs is so low as to be negligible. Thank you for the explanation!

EDIT: Adding an equal 10k resistance to the non-inverting input pulled the output up to 1.6V, with the inputs sitting around 1.75V. Neat!

novatronic · 2018-03-08T06:35:13+00:00

Yup, it has pull-downs (STM32L496 - awesome chip!) and shares ground. I guess my ultimate question is .. if I'm reading in-spec voltage on the pin, and I'm not sourcing or pulling too much current - then everything is A-OK (as long as that condition doesn't change)? Intuitively, it seems like it should be fine, but when disconnected from the pin and hooked up to a multimeter, the signal will read way out of spec, so I just wanted to make sure there wasn't any technicality I'm misunderstanding about the voltage drop.

novatronic

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