LTC2057 output offset - Page 1
Forum section: electronics / general

author: Bgdn
author_rank: Contributor
timestamp: March 09, 2023, 05:58:59 am
content: Hello,i have made the circuit from the picture bellow.The input voltage comes from ad8221, and the output voltage is feeding a adc ( ltc2368-24).The power supply for V+ is 3.3V, and for V- is -2V.Input signal is DC.My problem is that i have a large amount of offset voltageoutput of LTC2057 which is aprox 51mVI measured 1.4415 volts at the entrance ( V3 in the schematic), and on the output i get 1.3828Volts.Why is this offset? I am missing something here?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2023, 08:30:42 am
content: The output load may be too capacitive - so too little series resistance to isolate it from the input. With the low impedance output the layout (especially the ground) can be a factor too.So there is a chance the OP-amp oscillates or at least has strong ringing after the internal switching spikes and spikes coming from the ADC.At least for a test one could increase the 10 Ohm to let's say 100 ohm.The supply is rather low, but should normally be OK. With only some 5 V there are more alternative to the LTC2057, that may have better tolerance to capacitive loading.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 09, 2023, 08:53:09 am
content: The output might oscillate very probably, please check.Remove the 4n7 capacitor from the output.Frank

author: dietert1
author_rank: Super Contributor
timestamp: March 09, 2023, 09:34:21 am
content: The schematic does not show buffer caps on the supplies. Sometimes buffer caps help with capacitive loads.Regards, Dieter

author: Bgdn
author_rank: Contributor
timestamp: March 09, 2023, 09:38:43 am
content: Hello,the power voltage rails are buffered with 100n & 10uf caps.The pcb is a 4 layer one, with a solid ground layer, with no loops on the other layersI know the adc sar, has some spikes durring aquisition.I will try tonight to do a debug with a scope on the lines to see if i have some oscilations.I will keep you updated.

author: Bgdn
author_rank: Contributor
timestamp: March 09, 2023, 08:15:14 pm
content: Hello,i have mounted the scope at the entrance of the adc, there was an oscilation of about 50mv ( unfortunatly i did not found the stick to export the scope's picture).I removed the 4n7 capacitor, and the oscilation disapeard.Everything seems to work fine now.By the way, can you recomend some low ripple power supplies for negative/positive rails?In my design i use XC6210B332MR for the 3v3 rail, and LTC1550LCMS8-2 for the negative voltage.

author: MiDi
author_rank: Frequent Contributor
timestamp: March 11, 2023, 05:35:35 pm
content: Quote from: Bgdn on March 09, 2023, 08:15:14 pmLT3045/3094

author: Andree Henkel
author_rank: Regular Contributor
timestamp: March 14, 2023, 02:50:54 pm
content: 10 Ohms is indeed at the low end, values for the isolation resistor are 10..100circuit ca be improved with double feedback:problem with starting circuit is, that ADC mesurement value will change with sample rate: rising sample rate -> drop in result valuethis ca be cured with feedback from the reservoir capacitor instead of OP-Output, but with a resistor, typically 1kbut now you also need a feedback capacitor from OP output, typically about/less 1/100 of reservoir capacitor value.