LM2907-N: C2 Calculation

Hi Jeff,

have you also read this application note?

www.ti.com/.../snaa088

What is the LM2907-N doing in your circuit?

Kai

Hi Jeff,

Let us know if you still have questions after reviewing SNAA088.

Regards, Thomas

Precision Amplifiers Applications Engineering

In response to both questions above:

What is the LM2907-N doing in your circuit?:

LM2907-N is used to convert a variable frequency flow meter output (0 - 1.8khz) to an analog output (0-5VDC)

It is configured as described in figure 3 of snaa088 with a voltage divider on the output
C1 = 1nF
C2 = 1uF
R1 = 70K ohms

Have you also read this application note? www.ti.com/.../snaa088

Yes I have read the application note.

Page 4, Section “Choosing R1, C1 and C2” describes the process of selecting C2 in regards to ripple voltage Pk-Pk with the following explanation:

“It appears R1 can be chosen independent of ripple, however response time, or the time it takes VOUT to stabilize at a new frequency increases as the size of C2 increases, so a compromise between ripple, response time, and linearity must be cosen carefully”


Unfortunately this still does not answer my original question of response time, nor does it provide a graph or example I could use as a reference to derive the response time.

Hi Jeff,

Unfortunately, the only information we have about the LM2907-N is the datasheet and the AN-162 Applications Note, which doesn't address the response time to any depth. I've attempted to find a way to calculate it, but that would be a lengthy process of an unknown outcome.

Probably the best way to get to a response time number is to measure it. I have a test board designed for the LM2907-N, but need to have some fabricated. If you can wait a couple of weeks, I can have some boards fabricated and then do bench measurements.

You mention a 0 to 5 V analog output, I presume that is the input step you need the response time information for? Also, what is your circuit's supply voltage?

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Jeff,

Do you still need assistance with your LM2907-N application? We have had some PC boards fabricated that accommodate the LM2907-N and can do some reasonable testing. We would need to know the specific LM2907-N package type you are using, the exact circuit configuration, the supply voltage applied to the circuit, and the input voltage step for which you need the response time.

If you have progressed beyond this point with the project development and no longer need the information, please let us know.

Regards, Thomas

Precision Amplifiers Applications Engineering