| Electronics > Projects, Designs, and Technical Stuff |
| Very low bias current Op-amp to buffer a Kelvin Varley divider |
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| Kleinstein:
The LTC1052 is one of the few AZ OPs that still use external capacitors. This can help to get a low bias because of the low internal switching frequency. So there may not be a much better solution available. Using an external bootstrapping circuit can help to overcome the limit on the supply voltage. A lower supply may actually help to reduce the bias with some devices (the LTC2057 seem to be an exception). A low voltage also reduces power in the amplifier and thus thermal problems. Cooling seems good at first sight, but can get tricky, because of humidity and thermal EMFs. |
| splin:
--- Quote from: Gyro on June 14, 2015, 07:37:47 pm --- --- Quote ---You have to wonder if the author actually built that circuit and tested it or just did a theoretical design. I wonder how stable it is and how low you can reliably cancel the input bias current? --- End quote --- In their early dvms (even as far as the 106x), Datron routinely compensated bipolar op-amps (LM312) and matched transistor pairs (typical 3nA bias) to achieve bias currents of <50pA long term stable and drifting at <2pA/C. They used a combination of bias current compensation and bootstrapped supplies. I wonder if, with modern precision bipolar op-amps it might be possible to achieve much more stable results using similar compensation methods than the highly temperature sensitive bias currents of FET devices. Albeit with considerably more effort. --- End quote --- That's very interesting. Do you have any links to circuit diagrams? |
| splin:
--- Quote from: DiligentMinds.com on June 15, 2015, 03:13:52 pm ---The Fluke 720A only has 7 decades, because at 0.1ppm you are *already* in "Thermal EMF" trouble. At 70K output impedance, and with a 10V input to the 720A, 0.1ppm is about 14pA, so anything with 1.4pA or less should work, but I suspect you will have more troubles with Thermal EMFs at this level than you will with bias currents... To get really low bias currents *and* very low drift, you may be looking at a hybrid amplifier [dual-JFET input with chopper circuit to remove drift]. It think that there was a Linear-Tech ap-note on just such a thing. --- End quote --- You're probably right - will start looking... --- Quote ---For a monolithic opamp, the closest thing will likely be an LTC1052A, but this has only 18V max rail [not good if you are going from +/-10 input]. If you limit the voltage input to the 720A to the common mode restrictions of the LTC1052A, then that looks to be your best bet. To my knowledge, no other manufacturer has built a better ZD opamp with this low of a bias current and this high of a voltage rail [18V]. Note that the LTC1052A has quite a bit more input noise than the LTC2057, so if you are looking for less than 0.1ppm noise, you might compromise on the bias current and use an LTC2057 for that reason, and then just compensate for offset [since the LTC2057's bias current is rather stable to about 70C]. The LTC2057HV has the same specs, but is guaranteed to work with a 60V rail-- so it would allow much higher input range on the 720A for example. --- End quote --- The 2057 does have much higher current noise at 170fA/sqrt(Hz) compared to .6, but that only increases the noise to approx 300nV pk-pk with 70k source resistance so still much quieter in this case. CMRR is also 10dB better so it does seem to be a good choice, particularly if the bias current could be reliably compensated. The high voltage ability may not be much of an advantage though as even 150dB of CMRR equates to .032ppm at 10V and 150dB finite gain error another .032ppm, .064ppm total which is close to the KVD's .1ppm so bootstrapping may be required anyway. Do you have any suggestions, or places to look for very low leakage input protection solutions? When I've looked in the past it seems to be not so easy to find suitable devices. --- Quote ---If the LTC1052A/LTC2057 solutions are not good enough for you, then you are off to see the wizard [Jim Williams] about a hybrid discrete+opamp+chopper solution... --- End quote --- It may come to that, but I'm interested to see how Datron compensated for Ibias and if similar techniques coiuld be applied to the LTC2057. |
| Kleinstein:
Using the LTC2057 and bias compensation might be a good solution. Low leakage diode could be something like the BA199, some JFETs (gate to source+drain), small area photodiodes or just the collector - base junction of small transistors. Here also some degree of bootstrapping can help to limit the voltage over die diodes and thus really bring the leakage down. Photodiodes might also be used to compensate the bias, but even normal diodes in a not perfectly opaque case (e.g. BA199) might be sensitive enough for this purpose. |
| Gyro:
--- Quote ---That's very interesting. Do you have any links to circuit diagrams? --- End quote --- I do if you're prepared to squint a bit! I've never found a copy online, I meant to scan the whole manual sometime but my trial copy of Scan-n-Stitch expired (they're A3 schematics). Attached are the input board schematic, including i/p protection (Datron used a combination of JFETs, Zeners and an 88k 8W series resistor chain!), Ib compensation and bootstrapped supplies. Q11 (TO92) is thermally coupled (superglue) to the LM312 (TO99) under a plastic cover. The double sided PCB copper is arranged to be as isothermal as possible around them. All resistors are carbon film (apart from the high >10Mohm ones which are carbon composition). As I say, they managed to achieve >10G input resistance, <2pA/'C and 0.2uV/'C using VERY humble parts by today's standards and with no cpu to do auto-zero or correction in these units. My unit holds within 1-2uV from cold to operating temp on the 10mV range (x100 gain) with no appreciable long term drift (well aged by now!) The next generation - 1065/1061 are all evolutions of the same basic starting point, but with a cpu for compensation, autocal, autozero etc. I've also attached the relevant page from the circuit description. Hope it's at least of historic interest anyway. |
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