LMC7101: LMC7101 output signal is abnormal

Hey Rain,

Could you please provide a picture of the sticker from both orders of the devices?

Best,
Jerry

Hey Rain, 

Could you please share the schematic? 

All the best,
Carolina

Hi Caro

      Pls Check，Thanks.

Rain,

Could you also please provide a picture of the sticker from both orders of the devices?

Best,
Jerry

Hi Rain,

unfortunately, your circuit isn't stable:

Applying the dual feedback method can enhance the stability:

rain_lmc7101.TSC

Kai

Hi Rain,

and the aquisition time of ADC should be more than 40µs:

rain_lmc7101_1.TSC

Kai

Hi Kai

     Thanks for your support;

1. Why are (- IN) and - IN and (+IN) and+IN in the circle not directly connected? The purpose of doing this is not understood, please help guide?

2. What is the role of L2 and L3 in the figure?

Customer's circuit changes:

The customer changed C4 in his schematic diagram from 1nF to 330pF, and R3 increased from 100 Ω to 330 Ω. The shock disappeared, but he found that the interference in ADC increased.

Hi  Jerry

    Due to a long time, it is difficult to provide at present, so the customer has been asked to continue to find.

Hi Rain,

this is a setup to simulate the phase margin of the OPAmp circuit, to find out whether a circuit is stable or not. The role of L2 and L3 is explained in detail in this TI's training video series:

https://training.ti.com/node/1138805

What ADC is the customer using? Usually, the datasheet of ADC recommends a suited charge kickback filter (R3, C4) at its input. From this a suited OPAmp can be chosen which is able to properly drive this charge kickback filter.

Kai

Hi Kai

      thanks  for your support;

      The ADC use ADC104S of TI.

Hey Rain, 

Could you also please provide a picture of the sticker from both orders of the devices?

All the best,
Carolina 

Hi Rain,

the specified minimum clock frequency of ADC104S021 is 800kHz and the maximum aquisition time is three periods of 800kHz which is 3.75µs. So, the LMC7101 seems to be not fast enough for this ADC.

Kai

Hi Kai

1. LMC7101 is not fast enough for ADC104S201. What bad results will it produce?

2. What determines the speed of LMC7101?

3. How do you get the ADC acquisition time that you replied earlier is greater than 40us?

Hi Rain,

the 40µs minimum aquisition time is the result of the transient analysis I have performed for you. See here, with zoom:

And the consequence of a too slow OPAmp is inaccurate ADC readings.

Kai