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| Diffrential amp design |
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| ali_asadzadeh:
Thanks for the good feedback, I want to replace it completely, so I'm not bond to any part or voltages, Just something came to my mind,Can I trough it all out and replace it with a decent 24Bit differential ADC, with an internal gain of 128 or more? something used for wight scales but with better common mode rejection. since I just want to measure a uV signal between two probes technically. I want to achieve a good reading, either trough discrete op-amps, or better AMPs or another ADC, for Now I'm using AD620 and LPC1758 ADC to read the values, I know the LPC1758 ADC sucks, and it's not good enough, But we have made lot's of progress in recent MCU specially from ST and NXP, |
| OwO:
depends on your requirements: - CMRR - Gain - Noise floor (input referred) - Input common mode voltage range - Input differential mode voltage range - Input impedance - Bandwidth In some cases cheap opamps will do. I don't think abusing the differential input of an ADC is a good idea though. I've never seen CMRR specified on an ADC. I also haven't seen an ADC (apart from audio ADCs) that have that much built in gain. |
| Evan.Cornell:
--- Quote from: OwO on January 27, 2020, 06:38:20 am ---I also haven't seen an ADC (apart from audio ADCs) that have that much built in gain. --- End quote --- A specific use-case, but ADS1232 (http://www.ti.com/product/ADS1232) has built-in gain of 128. OP hasn't said bandwidth required, so I have no idea if this would be suitable for the application or not. |
| Zero999:
--- Quote from: OwO on January 27, 2020, 02:50:20 am ---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. --- End quote --- Sorry, I can't see how, without using very well-matched resistors, which would be expensive, unless there's something obvious I've missed. --- Quote from: MasterT on January 27, 2020, 12:30:32 am --- --- 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? --- End quote --- Yes auto-zeroing amplifiers theoretically have no 1/f noise below a certain point, but they do have more noise at higher frequencies, thanks to the interference from the oscillator. The data sheets don't seem to be showing it though, because as you said, there's no 10mHz to 1Hz data. --- Quote from: ali_asadzadeh on January 27, 2020, 06:27:17 am ---Thanks for the good feedback, I want to replace it completely, so I'm not bond to any part or voltages, Just something came to my mind,Can I trough it all out and replace it with a decent 24Bit differential ADC, with an internal gain of 128 or more? something used for wight scales but with better common mode rejection. since I just want to measure a uV signal between two probes technically. I want to achieve a good reading, either trough discrete op-amps, or better AMPs or another ADC, for Now I'm using AD620 and LPC1758 ADC to read the values, I know the LPC1758 ADC sucks, and it's not good enough, But we have made lot's of progress in recent MCU specially from ST and NXP, --- End quote --- What's wrong with the circuit I posted? It did get two likes after all. ;) You could replace the AD620, with the INA821, which is better in nearly every way and slightly cheaper, according to Mouser. It also has the same gain equation, so the resistor values wouldn't need changing. http://www.ti.com/lit/ds/symlink/ina821.pdf I can't say much about the ADC, as it's not something I've done before, but having a stable, low noise, voltage reference is very important. |
| ali_asadzadeh:
--- Quote ---A specific use-case, but ADS1232 (http://www.ti.com/product/ADS1232) has built-in gain of 128. OP hasn't said bandwidth required, so I have no idea if this would be suitable for the application or not. --- End quote --- The Sample rate of the signal is around 500SPS and the signal is an alternating signal in the 30Hz range, it has a shape like this --- Quote ---What's wrong with the circuit I posted? It did get two likes after all. ;) --- End quote --- It's a good Idea and thanks for sharing it, But maybe we could achieve something cheaper at the end of the day. |
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