Electronics > Metrology

LTC1052 + LTC1028 noise control ? amplifier compare to LTC1043 solution

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Overspeed:
Hello

As I search a easy ( not too costly not hard to locate IC ) solution to built an measuring amplifier DC for resistivity measurement

I have located these  schematic

Question 1 : does that a real improvement compare to the LTC 1052+LTC1043 solution ? I have never used a LTC1043 or similar circuit

Question 2: low noise measurement need skilled designed PCB , does some one have pictures ?

Regards
OS

Kleinstein:
The combination of the LTC1052 and LT1028 is used to combine the 2 amplifiers, especially use the LTC1052 for the very low frequencies where the 1/f noise of the LT1028 is higher.
However this solution does not improve the current noise of the LT1028 and is thus only suitable for low impeadance sources (e.g. < 300 Ohm, ideally < 30 ohm at low frequencies as the current noise also has quite some 1/f part). So it is not good together with the LTC1043 switched capacitor part.

The LTC1043 part is a way to get relatively good CMRR - still not perfect due to parasitic capacitance and charge injection that depends on the voltage.
For a resistance measurement one should avoid the need for a super good CMRR and thus way get away with a simpler circuit, like a more classic INA.
The resistance range (mOhm or Mohm) can also make a difference. The low and high resistance range may want different circuits and amplifiers as different problems get limiting. So there one circuit fits all.

Low noise not necessary needs that much care with the PCB. The layout is more a thing about EMI and precision (e.g. leakage currents or thermal EMF to cause errors).

Overspeed:
Hello

My need is to measure low value to very low resistance for a 4 points system at macro scale so I want to keep current low as possible and measuring voltage will be low . I need an ''high precision amplifier system '' as I need to high light difference between samples and not lost my signal in noise or other problems .

I work in DC only will be pulsed at low frequency and switched polarity for future trials , but first need is a true microvolt amplifier DC .

Last parameter is if possible to stay in DIP or SOIC foot print to avoid soldering problems .

Thanks in advance

Regards
OS

Kleinstein:
When testing resistors the common mode voltage should not be very large. So there is no need for the LTC1043 type circuit for really good CMRR.

For low noise there are now low noise AZ OP-amps (e.g. OPA189, ADA4522) and there is less need to combine a AZ with a normal OP-amp. With most DMMs / ADCs one would not really care about the higher frequencies anyway.  The choice with DIP case is limited but the 2 OP-amps noted are available in SOIC8 case.

The accuracy may be limited by thermal EMF anyway. The OP-amp noise is more like a smaller problem.  A little more current can really help. Similar AC excitation or polarity reversal at the low frequency can also help, not just with the amplifier but also thermal EMF problems.

KT88:
The main intention of combining the LTC1052 with the LTC1028 is to overcome the bandwidth limitations of AZ amps. One major issue with AZ amps is the once signal and noise exceeds fclock/2 aliassing- and intermodulation artifacts could degarde the performance at low frequencies significantly. Another one is that with higher gain (which is desired in most use cases) the bandwidth of AZ amps is very limited.
Since in this case bandwidth seems to be no issue a recent AZ amp would do the trick. The ones Kleinstein mentioned are fine.
If the noise performance isn't still satisfying, a parallel configuration could help. The noise would decrease by 1/sqrt_N.
For a low source impedance the ADA4523 might have an edge over the aforementioned AZ amps...
A handy tool collection could be found here: https://tools.analog.com/en/precisionstudio/. You can enter your design and run a noise analysis for different amplifiers.

Cheers

Andreas

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