Hello,

I'm trying to use AD650 as a frequency to voltage converter.

Requirements:

Fin max = 1MHz, Voltage =0-10V.

From the calculator on the website I obtained Rin=11,11KOhm, Cint= 1.44e-5, Cos=8.824e-11. So I applied Rin=11,5kOhm, Cint=82pF, Cos=15uF.

The problem is, that during measurements, the output voltage changes in weird way, by what I mean:

1MHz 9.4V

900kHz 8.4V

800kHz 7.4V

700kHz 6.5V

600kHz 5.6V

500kHz **9.5V**

400kHz **7.5V**

300kHz **5.6V**

200kHz **5.6V**

100kHz **5.6V**

90kHz **5.89V**

80kHz **5.21V**

70kHz 4.5V

60kHz 3.9V

50kHz 3.2V

40kHz 2.72V

30kHz 2.06V

20kHz 1.4V

10kHz 0.6V

9kHz 0.593V

8kHz 0.527V

7kHz 0.458V

6kHz 0.393V

5kHz 0.331V

4kHz 0.268V

3kHz 0.205V

2kHz 0.138V

1kHz 0.070V

900Hz 0.063V

800Hz 0.060V

700Hz 0.053V

600Hz 0.045V

500Hz 0.040V

400Hz 0.030V

300Hz 0.023V

Does anybody have an idea what's going on? And why the results are so weird? Is this satndard behaviour? Hope not...

Especially,

1. Why there is a voltage jump up to the max voltage output reserved for max frequency?

2. Why the voltage is stable between 300kHz and 100kHz?

3. Why the voltage rises in 90kHz, when it should decrease?

I really need somebody's help

Kate

Hi Kate,

The average voltage output are a factors of one shot period, Rint, current source and Fin. Since the one shot period is determined by Cos, together with Rint they are both computed. For now I am thinking of the current source, which is base from the block diagram it is connected to the -Vs. I suggest to check the power supply, because this affects the conversion. Although it looks weird that it only happened on these specific frequency range. Can you share your schematic so we may have a better understanding of the possible problem. You may refer to the Application note AN-297 http://www.analog.com/media/en/technical-documentation/application-notes/75729603AN-279.pdf

Regards,

Jonathan