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| Diffrential amp design |
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| MasterT:
--- Quote from: Zero999 on January 26, 2020, 08:10:32 pm --- --- Quote from: 741 on January 26, 2020, 07:48:24 pm ---"INA333 Micro-Power (50μA), Zerø-Drift, Rail-to-Rail Out Instrumentation Amplifier" :-+ --- End quote --- It's noisy though, compared to the AD620 at 50nV/√Hz, vs 9nV/√Hz, which I think could be an issue as they're dealing with µV level signals. --- End quote --- What I see, ad620 noise goes up to 300 nV in the sub-Hz frequency. Application has filter at 0.4 Hz bandpass, so only this band matters. I'd agree that AZ OPA is better option in this case, since noise flour stays flat for them. Logically, AZ circuitry just "null out" low frequency noise and becomes a winner. |
| Zero999:
--- Quote from: MasterT on January 26, 2020, 10:00:47 pm --- --- Quote from: Zero999 on January 26, 2020, 08:10:32 pm --- --- Quote from: 741 on January 26, 2020, 07:48:24 pm ---"INA333 Micro-Power (50μA), Zerø-Drift, Rail-to-Rail Out Instrumentation Amplifier" :-+ --- End quote --- It's noisy though, compared to the AD620 at 50nV/√Hz, vs 9nV/√Hz, which I think could be an issue as they're dealing with µV level signals. --- End quote --- What I see, ad620 noise goes up to 300 nV in the sub-Hz frequency. Application has filter at 0.4 Hz bandpass, so only this band matters. I'd agree that AZ OPA is better option in this case, since noise flour stays flat for them. Logically, AZ circuitry just "null out" low frequency noise and becomes a winner. --- End quote --- That's what you might think, but the data sheets say otherwise. https://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf http://www.ti.com/lit/ds/symlink/ina333.pdf Input voltage noise from 0.1Hz to 10Hz is 1μVPP for the INA333 and 0.28μVPP for the AD620, at gains above 100, which the original poster's design has. The only advantage the INA333 has, noise wise is lower input current noise at 2pAPP vs 10pAPP, so might be better if the input is high impedance, but it's only rated to 5.5V single supply and the original posters design is +/-5V. |
| GigaJoe:
I'm wonder ... if alternatives was observed ... as an example from the first page of AD620 PDF ? like AD8221 ? or replacement by regular 3-opamp schematics, with much wider choice for a regular opamp ... |
| MasterT:
--- Quote from: Zero999 on January 26, 2020, 10:54:48 pm ---... That's what you might think, but the data sheets say otherwise. https://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf http://www.ti.com/lit/ds/symlink/ina333.pdf Input voltage noise from 0.1Hz to 10Hz is 1μVPP for the INA333 and 0.28μVPP for the AD620, at gains above 100, which the original poster's design has. The only advantage the INA333 has, noise wise is lower input current noise at 2pAPP vs 10pAPP, so might be better if the input is high impedance, but it's only rated to 5.5V single supply and the original posters design is +/-5V. --- End quote --- The problem with DS is that there is no 0.01 - 1 Hz line, and looking into similar IC from TI TLV333 , where noise spec is split into 0.01 - 1 and 0.1 - 10 ranges, I see 0.3uV and 1.1uV, that's more than x3 times. Right, it's just analytics, but if I go father into <0.5 Hz than difference with ad620 where line is sharply rising would be more obvious. Theoretically, I don't know if China have any part numbers, very unlikely anyway. Regarding voltage, OP was going to redesign circuits and why keep same voltage? |
| OwO:
Oh I didn't realize it's using a gain > 100. In that case a discrete design with 3 opamps can achieve >100dB CMRR easily because the first stage provides 40dB of common mode rejection. |
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