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| Dual slope ADC improvement |
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| Kleinstein:
If the 4053 switch is used for the integrator reset the integrator Output swing would be limited to the supply fo the 4053 and thus some +7.5 V and -2.5 V. With a positive input (7.5 V) the integrator output would swing negative. So the -2.5 V limit would be relevant. It would probably be better to have a more symmetric supply, like +-5 V and limit the input range to +5 V so one also has 5 V to room to the negative side. Alternatively one could limit the voltage between the integrator output and switch for the integrator reset, so that the integrator could swing more than -2,5 V. This usually introduces some resistance and thus would slow down the reset phase a little, but not to bad. Using a zero input conversion for auto zero is possible. The effect is a little different from an AZ OP: it compensates essentially all offsets, but is slower. So there can be some 1/f noise as the cycle takes quite some time (e.g. 20 ms integrate, 60 ms disintegrate, 5 ms cap reset and the whole 2 times). At the planed resolution it can be still acceptable with a suitable OP. An integrator offset would also effect the reference. Relative to a 2.5 V reference an 25 µV error would not be that bad, though possibly visible. |
| namster:
--- Quote ---If the 4053 switch is used for the integrator reset the integrator Output swing would be limited to the supply fo the 4053 and thus some +7.5 V and -2.5 V. With a positive input (7.5 V) the integrator output would swing negative. So the -2.5 V limit would be relevant. --- End quote --- I completely reversed the operation of the ADC , that's it the 7.5 input will be inversed to negative value that will cause saturation problem in switch ! --- Quote ---It would probably be better to have a more symmetric supply, like +-5 V and limit the input range to +5 V so one also has 5 V to room to the negative side. Alternatively one could limit the voltage between the integrator output and switch for the integrator reset, so that the integrator could swing more than -2,5 V. This usually introduces some resistance and thus would slow down the reset phase a little, but not to bad. --- End quote --- the major probleme is that i have designed a current source that is supplied by +-15 Volt ! i have to review the all design a correcte that for a single supply for all the instrument , is a linear supply less noisy than a modern Switch mode supply ? --- Quote ---Using a zero input conversion for auto zero is possible. The effect is a little different from an AZ OP: it compensates essentially all offsets, but is slower. So there can be some 1/f noise as the cycle takes quite some time (e.g. 20 ms integrate, 60 ms disintegrate, 5 ms cap reset and the whole 2 times). At the planed resolution it can be still acceptable with a suitable OP. An integrator offset would also effect the reference. Relative to a 2.5 V reference an 25 µV error would not be that bad, though possibly visible. --- End quote --- do you have an example of AZ opamp , is the ICL7650 a good choise ? |
| Kleinstein:
A linear supply can be lower noise than a typical switched mode. However a well designed switched mode supply can be good enough. There are just more point's that can go wrong with a switched mode supply. For the first test there is likely no need to use a SMPS: the supply current should be relatively low and thus a linear regulation should be acceptable. The IC7650 is an old AZ OP, and OK. Some modern low cost ones are MCP6V27 and similar. The AZ OP does not need to work with a high supply. There is no absolute need for an AZ OP. A good quality normal OP and using the zero reading can be good enough. Also an AZ OP does not solve all drift problems: there is still drift from leakage currents and the buffer amplifier. How does the circuit to the PCB look like ? Maybe it does not take so much change and the board could still be used. For a little higher supply there is the old and slightly slower CD4053 / HEF4053, that allow a little higher supply (up to 20 V in the B Version). |
| namster:
--- Quote ---A linear supply can be lower noise than a typical switched mode. However a well designed switched mode supply can be good enough. There are just more point's that can go wrong with a switched mode supply. For the first test there is likely no need to use a SMPS: the supply current should be relatively low and thus a linear regulation should be acceptable. --- End quote --- i will try to do a linear PSU for the analog part of circuit and a SMPS for digital and auxiliary psu to start the instrument from a button in front panel --- Code: ---How does the circuit to the PCB look like ? Maybe it does not take so much change and the board could still be used. For a little higher supply there is the old and slightly slower CD4053 / HEF4053, that allow a little higher supply (up to 20 V in the B Version). --- End code --- there is several circuit in first version , i will draw a new pcb that containt differents part of instruments ,thank you for all the interest you have on this subject |
| Kleinstein:
For the first test it should be good enough to use a solder-less bread board. No need to design a PCB so early - the final PCB version may give a slightly lower noise and better linearity, but usually not that much. For my multi-slope version I started with a bread board for the analog part and an small reused PCB similar to the Arduino. One can also start with simple parts (e.g. cheaper OPs) and simple resistors. The first step is to get a suitable schematics and at least a rough idea of the software. The software can also limit the timing resolution. For a Ohms meter one may not need negative values, at least not much range to the negative side. Another question is if oone needs /wants a (quasi) differential measurement. It can help to draw not just one solution, but also a few alternatives to compare them (expected performance, size/cost and software requirements). The first round can be just from the drawing, maybe with a simulation if needed. Chances are one can decide on paper which solution is suitable. |
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