How much opamp leakage is acceptable for polymer film piezo preamp - Page 1
Forum section: electronics / projects

author: Balthazar
author_rank: Contributor
timestamp: April 16, 2022, 04:10:29 pm
content: Hi,I’m considering building an electric cello as a hobby project. I want to try out polymer film piezo pickups, for they are noted for flat frequency response and linear dynamic range. That means higher voltages than a regular ceramic piezo, but also much smaller charges. Piezo transducers can be seen as perfect AC voltage sources, in series with a capacitor. For ceramic piezo discs, that capacitor may be 10-20 nF, for the polymer film product I’d like to buy, it’s only 480 pF. I’ll probably put two in parallel to get 960 pF. That means I’ll need a high input impedance in the preamp, 10 or 20 MOhm depending on whether I put two transducers in parallel or not.In this interestingread, made by the manufacturers, it says that I might want a "low leakage opamp" to prevent "errors in measurement". Does "low leakage" basically mean the same as low input bias current? I see all the opamps they suggest are low input bias current, some even in the fA range. Do I really need this, as most of those opamps are much more expensive. This guyright heresays he got very good results by going from the cheap tl071 to the also cheap NE5334. He used ceramic piezos, which have much more capacitance, but only by about an order of magnitude. The NE5334 has huge amounts of input bias current compared to what this paper considers "low leakage". I’ll probably use his schematic as a starting point.Not sure what else you need to know, as I don’t know what might happen if my opamp is too "high leakage". I think the voltages will be in the order of 100’s of mV to a few volts, but can’t be sure untill I’ve ordered and tried it out. Charges on the order of 100’s of fC to a few pC. Before I order parts and get to find out, I’d like to know whether I need to care about leakage/input bias current, and how much I can accept.

author: Marco
author_rank: Super Contributor
timestamp: April 16, 2022, 04:43:20 pm
content: Low bias current is for BJT opamps, low leakage is for CMOS/JFET opamps. The leakage in and of itself is not really an issue for AC amplifiers, as long as you don't forget the current needs somewhere to go. High leakage generally also means high current noise though.That's what you should focus on, low current noise. Which is to say low noise JFET or CMOS opamps.

author: TimFox
author_rank: Super Contributor
timestamp: April 16, 2022, 05:02:48 pm
content: Also, note that some op amps achieve very low input DC currents (bias or leakage) by an input stage that "mirrors" the bias current and adds the negative of it to the input, thus approaching 0 DC value, but the current noise superimposed on that DC value increases by 21/2, the sum of two equal uncorrelated random values.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 16, 2022, 05:09:01 pm
content: In the audio range the 500-1000 pF source has an input impedance of around 1 M and lower for the higherst frequencies. So the opbvious choice is an OP with FET input. The TL071 may be a bit noisy, but in principle OK. The NE5534 or OP27 would be a bad choice, even with a 10 x higher capacitance.Good modern FET types would be something like OPA172, OPA134, OPA1641, AD8510. Chances are the choice is not that critical.

author: magic
author_rank: Super Contributor
timestamp: April 16, 2022, 05:48:13 pm
content: Reactance of 15nF at 1kHz is 10kΩ. With 0.6pA/rtHz current noise of NE5534, the result is 6nV/rtHz or 6dB more than NE5534 voltage noise and 10dB better than TL071.It gets 20dB worse (and almost* 10dB worse than TL071) at 100Hz, but hearing sensitivity also falls down.With only 1nF capacitance, NE5534 noise will be 13dB higher and no longer competitive even at 1kHz.Beware that most of TL071 and OPA134 on auction sites are fake. The former are often one half of LM358, a fairly noisy bipolar opamp. OPA1641 is nice, but out of stock everywhere last time I checked.*TL071 voltage noise increases at low frequency, see the datasheet. Beware of TI's new TL071H, which is a CMOS opamp with more audio band noise.

author: Marco
author_rank: Super Contributor
timestamp: April 16, 2022, 06:15:47 pm
content: OPA141 seems easily available.

author: Balthazar
author_rank: Contributor
timestamp: April 16, 2022, 06:54:28 pm
content: Thank you for your replies. After playing around with it in SPICE, it appears that the noise generated due to current noise in the opamps are being lo-pass filtered by the capacitance in the transducer. For that reason, the 3fA per sq. Hz of the opa134 will be well within tolerance. I been eyeballing it already, it seems a good fit. The opa141 also looks good, but I could only find it with SMD mount on mouser where I’ll buy it. I don’t buy on auction sites because I can’t be sure of authenticity.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 16, 2022, 08:24:05 pm
content: The old style DIP package is rare with modern parts. With relatively cheap available professional made PCBs SMD should not be a problem. It needed there are also adapter boards for SO8 and SOT23-5/6 parts.

author: Balthazar
author_rank: Contributor
timestamp: April 17, 2022, 10:15:12 am
content: I think the last time I tried one of those PCB print services was 5-6 years ago. The quality was good, but it was still a bit tedious and expensive for a hobby project, when I could have just taken a prototype board and soldered it together in half an hour. Especially for experimentation, DIP always seemed much easier and quicker for me. Are there adapter boards which you could solder onto the prototype board (through hole) and then try out different SMD chips, like you can with a DIP socket? That would be great.

author: magic
author_rank: Super Contributor
timestamp: April 17, 2022, 10:50:20 am
content: There are small adapter boards where you solder the SMD part and two rows of pin headers (the round ones, not square). The complete assembly can be insert into sockets like a normal DIP chip.OPA134/2134 is probably the best widely available JFET opamp in DIP. There are lower noise ones like LT1792 but less common and more expensive.

author: Balthazar
author_rank: Contributor
timestamp: April 17, 2022, 05:36:50 pm
content: Good. I decided to ditch the opa134 for opa172. It has less durrent draw, which is good cause it will be battery powered. I tried to find the 171, but they only had it in a package they call vssop-8, which looks even scarier than soic-8 to me. I haven't ever soldered an smd part so I'll better start with something easier.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2022, 06:24:25 pm
content: If low power is important, one could consider the OPA197 (rel. low noise and still less current than the OPA172) or opa196 (noise comparable to TL071, but about 1/10 the power).

author: John B
author_rank: Frequent Contributor
timestamp: April 17, 2022, 11:38:51 pm
content: Do you have a link to which pickups you're using?

author: Balthazar
author_rank: Contributor
timestamp: April 18, 2022, 12:33:46 pm
content: Yes:https://www.sparkfun.com/products/9196I wouldn't exactly call them pickups, rather they are general purpose sensors which can be used for many other purposes as well as making pickups.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 18, 2022, 02:39:06 pm
content: The offset error is ~ Vos+(Rf*ib). So its a trade off between gain and offset. If your output is AC coupled then no problem use a regular low noise fet or cmos input amplifier. PCB and cleanlines are needed to make the most of ultralow bias amps.