LM2903: Input filter recommend for LM2903

Lacey Lin

Part Number: LM2903

Hi team,

previously you have helped to reviewed the schematic of the LM2903.

And it seems the schematic is OK. As below:

Now because there is some noise in the input signal, customer want to add a 2.2nF cap in the input pin to form a filter.

Can you help to comment if it's ok for LM2903, if not, what's your recommendation?

Lacey

Thanks a lot!

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Lacey,

where is the coming from? From RT? Is RT mounted outside of the circuit? Do you use shielded cables for RT?

Kai

0 Lacey Lin over 6 years ago in reply to kai klaas69

TI__Intellectual 2900 points

Hi Kai,
The LM2903 is used as a comparator to monitor the 12V supply voltage.
So the noise and ripple wave of the 12V supply voltage will have some effect.
So my customer want to add a cap to filter the noise and ripple wave.
Just want to make sure if the cap will affect the functional of LM2903.

Can you help to check?

Lacey
Thanks a lot!

0 kai klaas69 over 6 years ago in reply to Lacey Lin

Guru 140200 points

Hi Lacey,

hmm, are you sure that the 12V supply voltage shall be monitored? I ask, because the 12V supply voltage is given to both inputs of comparator. Are you sure that it doesn't have to do with RT?

You can add low pass filtering to the 12V supply voltage. But I would do it at a place where both inputs profit from it by the same amount. So, I would insert a RC filter to the most left of schematic.

What noise and ripple does the customer observe (frequency, amplitude)? Do you have a scope plot?

Kai

0 Paul Grohe over 6 years ago

TI__Guru 66706 points

Hi Lacey,

Can you clarify the purpose of the circuit?

It looks like it is a temperature monitoring circuit.

The 47k and 49k "RT" and "R?" resistors values do not make sense. Is RT supposed to be a 1K RTD? Is there one or two?

You can place a cap on the inverting node to filter the reference voltage, buy you cannot put a cap on the positive node because of the hysteresis feedback. The cap can probably be placed at the node between the RT's and 49.4k R?.

0 Jonathan Nguyen over 6 years ago in reply to Paul Grohe

TI__Expert 4555 points

Lacey,

Do you have anymore questions regarding the topic of this thread?

Regards,
Jonny

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LM2903: Input filter recommend for LM2903