Mixed op-amp amplifier (lt1151 and lt1028) hybrid problem - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: January 25, 2018, 02:29:55 am
content: I made the circuit on page 10 of this app notehttp://cds.linear.com/docs/en/application-note/an21f.pdfBut I used a LT1151 amplifier. I checked all the resistor values, and it is on a breadboard. The capacitor I used is a 11nF silver mica type.Now, the app note says that it should adopt the offset voltage of the LT1151, which is only typically 500 nanovolts.However, if the input is grounded, I measure 110milivoltson the output, which means the input offset voltage is 100 microvolts, since the gain is 1000.Whats the deal? I tried a few different LT1028's and two LT1151's. Changing the LT1028 varies the offset voltage somewhat, but I am two orders of magnitude off from where I should be.Any ideas?EDit: may be a breadboard problem. I wiggled the resistors around and it dropped to 50 milivolts offset. I gotta rebuild it with solder I think

author: David Hess
author_rank: Super Contributor
timestamp: January 25, 2018, 06:35:22 am
content: I have built this circuit on perforated board and it worked great but I used a very good layout. I also extended it to a differential version with 2 LT1028s and 2 LTC1150s. Parts selection is not critical however thermoelectric voltages from thermocouple junctions will completely destroy the low frequency low noise performance of this circuit. To prevent this, the circuit needs to be shielded from air currents and symmetrical circuits and layouts should be used which is covered in Linear Technology application note 9.I made measurements of exactly how much the offset changes due to current into the offset pins so I could adjust the gain to use the full range of the LTC1150. Then I trimmed the time constant of the integrator for minimum low voltage noise and the low frequency noise curves of the LT1028 and LTC1150 crossed exactly where they should based on the datasheets.The ultimate noise level was so low that I could use it to measure low valued resistors at the input based on their Johnson noise alone.There is a variation of this circuit which uses a discrete large area JFET input stage for both low voltage noise and low input bias current.

author: CopperCone
author_rank: Super Contributor
timestamp: January 25, 2018, 05:37:17 pm
content: You adjusted the gain by varying the 30k resistor? The 100k is just for isolation right?Am i right in thinking you can measure the output voltage of the chopper amplifier and adjust 30k so that its its close to 0v output on thr chopper to maximize the dynamic range across temperature?

author: David Hess
author_rank: Super Contributor
timestamp: January 25, 2018, 10:16:18 pm
content: Quote from: CopperCone on January 25, 2018, 05:37:17 pmThe gain which controls where the noise curves overlap is adjusted by any of the three elements, the input resistance, the integration capacitance, or the output attenuator. In practice the output attenuator is adjusted so that the LT1052 can capture the maximum expected offset voltage with some leeway, I think I used +/-10 volts for +/-500 microvolts of input offset or something which is very conservative, and then the combination of the input resistance and integration capacitance are adjusted to that the noise curves cross at a point to produce the lowest total output noise.The output range is not that critical but I did it anyway; it just needs to be insensitive enough so that the *output* noise from the LT1052 does not appear in the output of the LT1028. There may be other benefits to doing this like making the circuit more insensitive to outside influence. I had to do this anyway because I tested the circuit with different parts in place of the LT1028 and they had different adjustment pin sensitivities.If you know the adjustment pin current to produce an input offset voltage which is what I measured, then you can calculate the rest of the values for lowest total noise. After I did this, I verified that I had the right value of capacitance by substituting lower and higher values and measuring the 0.1 to 10Hz RMS output noise. I do not remember if the values given in the circuit were at that point but they were not far away and it is not very critical. I just wanted to know that it was working.QuoteYes, you could do this but I did not bother. The LT1028 and similar amplifiers already have a very low offset voltage and offset voltage drift so this is not necessary. The extra manual trimming circuit would not improve reliability.