USB 6008 weird analog voltage reading

[learnerd]

Hi,

i am using the USB 6008 to measure a voltage of a Lithium battery, 3.66V.

the battery come with a blocking diode( in series with battery ) of 1N5820, which have a forward voltage drop of 0.1V.

battery with diode in series ( that is how the battery comes with )

- measuring with DDM yield 3.66V without connecting to usb6008

 - measuring with DDM yield 3.59V connected to usb6008 ( USB6008 power OFF )

 - measuring with USB-6008 yield 3.59V connected to usb6008

battery with diode removed

- measuring with DDM yield 3.66V without connecting to usb6008

 - measuring with DDM yield 3.66V connected to usb6008 ( USB6008 power OFF )

 - measuring with USB-6008 yield 3.66V connected to usb6008

so, it seems that the problem is in the series diode.  why is the battery's voltage dropped 0.07V? will the series diode harm the USB 6008?

maybe the people that know the circuits inside the USB 6008 can give me an answer.

Thanks in advance.

[kellam · Accepted solution]

Hi learnerd,

The drop you are seeing is the forward drop in the diode.  As an entry class device, the USB-6008 has a relatively low input impedance (144kOhm), and as such is drawing a little bit of current from the device.  Looking at the datasheet for the 1N5820 (http://www.onsemi.com/pub_link/Collateral/1N5820-D.PDF), Figure 7 shows that at 25C, a draw of 50mA will cause a forward drop of 200mV.  While the figure doesn't extend the curve below 50mA, extrapolating from the given curve would indicate that a 70mV drop would be cause by only a few microamps of current.

A DMM will have a much larger input impedance (GOhms instead of kOhms), and won't draw enough current to affect the measurement, which is why you wouldn't see the drop with just the DMM.

The diode will not harm the USB-6008 in any way. 

Good luck with your application,

Jeff

[learnerd]

thank you for the quick response.