## Friday, September 20, 2013

### Measuring Injected Current and Input Bias Current on a DMM

I recently did a webinar (Web based seminar) for the Truevolt series of DMMs. A question came up that I couldn't answer right away so I promised the person to blog about it. The question was about measuring injected current on the DMM.  Since I didn't know how to do it off the top of my head I needed to do some research.

Truevolt's datasheet highlights the DMMs' low level of injected current but there isn't a line item on the spec sheet for what the injected current should be. So I thought that I would measure the injected current and show my results on this blog. Keep in mind that this is not a specification and I'm only measuring on one unit, but it should give you an idea of the levels of injected current that you can expect.

When I started my investigation I got confused on whether I should be measuring input bias current and did some research into that too. I'll speak about the differences below between the two currents and how to measure both signals on my 34461A. These techniques should be viable for any DMM that you have on your bench.

### Measuring injected current

Let's start off with the question from the webinar. What's the injected current level of a Truevolt DMM. You might be also asking yourself what is injected current. Injected current for a DMM is low level current that comes from the DMM earth referenced circuitry such as the transformer.  Since the DMM has floating inputs this current does not normally affect your measurements. However this current can be seen when your device under test's (DUT) signal is earth referenced as well. The injected current will affect your AC measurements if your signal is earth referenced or poorly isolated from earth ground.

We typically characterize the injected current as an AC current. If a DMM is designed poorly, then this injected current can introduce large errors on your measurements.

It turns out that the method to measure injected current is actually fairly simple for a DMM. The only connection that you need to make is to connect the high terminal to earth ground. Then setup your DMM to make ACV measurements.The connection is shown in this simple diagram. To make the earth ground connection, I took a BNA cable and connected it to the rear earth ground screw on the back of the DMM.

I then took a few readings and used the 34461A's internal statistics to show what my average and standard deviation are.
As you can see, the average was about 50mVrms with a standard deviation of 72uV. I did a quick check to see if the noise was normally distributed and it looked like a fairly normal Gaussian distribution.

In order to calculate the injected current from this we simply divide the ACV measurement by the input impedance of the DMM which is 1Mohm for this function. With this measurement method I can use the 1Mohm input impedance of the DMM in ACV mode as a high value shunt.

So for my DMM the injected current is about 50nA. I'm not sure if this is typical but I would guess that most units would have less than 100nA of injected current. These levels are quite good for a 6.5 digit DMM and Agilent believes that Truevolt DMMs are among the top of the class for this parameter.

### Measuring Input Bias Current

I mentioned above that I started investigating input bias current because I thought it was the same thing as injected current. It turns out that input bias current is floating current that flows in or out of your DMM. This current will affect not only your earth reference measurements but your floating measurements as well. Luckily this is fairly small in most cases and the Truevolt DMM have a characteristic of <30pA at 25 C.

There are two methods of measuring Input Bias current (Ibias). One method requires knowledge of the DMM's internal input capacitance while the other is a more straightforward but not as accurate version. I used the more straightforward method since I don't know what the input capacitance of my DMM is and you might not either if you are using a different type of DMM.

The more accurate version of measuring Ibias would be to calculate the time it takes the DMM to change from one voltage to another on a fixed range after initially shorting the input terminals. You then need the input capacitance to plug into the the following equation:

For example, if the DMM were to take 1 seconds to change from  0.775V to .9V. The deltaV would be 0.125V.  If we say that the input capacitance is 200pf (this is hypothetical) then

$Ibias = 200e-12*0.125/1 = 25pA$

Since I don't know the input capacitance and I'm not sure that I'd be able to get my timing down very well I'm using a different method to measure the Ibias. The more straightforward measurement would be to put the DMM into DCV and change a couple of settings:
- Set Input Z to 10M
- Turn of Autozero

The connection to the DMM is even more simple. Keep the terminals open i.e. take off any cable or signals to the DMM. By setting the input impedance to 10M it will effectively bleed off any residual charge that might be on the terminals. I had my terminals open for a while before I started taking readings.

So with the terminals open, I took a screen shot of a number of readings. The screenshot also shows the DCV settings that I have which include the two setting changes from above. My average is 9uV.

Then to calculate the Ibias current, all I need is a simple equation. With this measurement method I can use the 10Mohm input impedance of the DMM as a high value shunt.
$Ibias = V/Rinput$

So the resulting Ibias = 9uV/10Mohm = 0.9pA. This result is for one unit but it does show that the input bias current of Truevolt DMMs is meeting it's specification with quite a bit of margin.

1. We help in setting up and Installing Microsoft Office on your computer. We provide Technical Support and other services to you. We help you with great work and other details. We are here to help you. We help you with Microsoft Office and Office installation. We help you with Great technical services at an affordable price.
Regards- www.office.com/setup

2. I just want to say that all the information you have given here on is awesome. Thank you Orthopedic Hospital in Hyderabad

3. I would like to say Thanks for helping. Orthopedic Surgeon in Hyderabad

4. You have done very well its really helpful post for skinny head . if you are not satisfy to other .you can check this link and get your dental treatments. Bariatric Surgery in Hyderabad

5. I just want to tell you that I’m all new to blogging and site-building. I really loved your web page. Very likely I’m want to bookmark your site . You amazingly have remarkable posts. Thanks a bunch for sharing your blog.
Regards - www.office.com/setup
www.office.com/setup
www.office.com/setup

6. Sometimes Microsoft Office users face the issues after upgrading their Windows operating system. If you are also facing such type of issues related to your Microsoft Office setup installation then contact us at office.com/setup for the best support.
office.com/setup

7. McAfee.com/activate – McAfee Antivirus is one of the most critically acclaimed security program across the globe. Protect your devices and data from viruses, malware, Trojan horses, worms, and other cyber threats with real-time protection and threat scanning. Get robust protection offered by McAfee by following the process (download, install, activate) on  www.mcafee.com/activate

8. Microsoft Office is one amongst the most useful productivity suites for Mac and Windows. Log in at office.com/setup and browse various packages for Office 2019, Office 365, Office 2016, Office 2013, Office 2010 and more. After downloading and installing its setup, activate it with the product key from  office.com/setup

9. Thank you so much for sharing these amazing tips. I must say you are an incredible writer, I love the way that you describe the things. Please keep sharing.

For more information visit on office.com/setup | office.com/setup | office.com/setup | Norton.com/setup

10. Go to office.com/setup to purchase, download, install, and activate Office 2019, 365, 2016 and many more. Enter You Office Product key on www.office.com/setup to activate your Office setup.

11. While printing some important document, you may get an error message that HP Printer Offline and printer can’t print anything. At that situation, grab the quick solution from dexterous technicians via dialing our toll-free number instead of wasting time in wandering here and there. Hence, call on the provided number and bring your offline printer to online hassle-freely. https://www.hpprintersupportpro.com/my-hp-printer-is-offline

12. I am really influenced to read such a great post. It is requested to keep up this habit. You must go through link Assignment Help to reduce the hectic pressure.

13. Many users are successfully using Free Avast antivirus, but at some point, few of them are facing trouble due to installation of Avast antivirus on their system. This trouble becomes too critical and unable to handle by the own. In that situation, users immediately need to uninstall Avast Antivirus from their system. To do in a perfect way you need to contact tech experts for antivirus issues from getting a reliable Avast Customer Support and instant help as well. https://www.avastsupportnumber.us or https://www.youtube.com/watch?v=BpOK1vxQzCA

14. Thanks, great post. I really like your point of view. I m Marry Taylor, I working as a support executive for printer devices. I am offering support for HP Devices and If you need help then contact HP Tech Support Phone Number +1-888-902-8333 and you can also chat us to get assistance to fix issues regarding HP printer.

15. Brother Printer Support Phone Number team will help you with all your technical difficulties in using brother printer and will resolve all your doubts regarding the setup and installation process. calling the tech support team will save your time and money. so call us now on Brother Printer Support Phone Number +1-888-902-8333 to help you with your difficulties.

16. Brother printer gets offline suddenly because of the network connection problem. Therefore, if you are encountering the Brother Printer Offline issue, then don’t get panicked! Just check your cable connection. In case the cable is loose, plug it into the power-point tightly and then use your Brother printer. While printing, if yet you are getting an offline error message, then get united with the dexterous technicians. They will help you out within the least time interval.

http://www.webroot-webroot.com

18. norton.com/setup to install setup product key, Learn how to download, install, and activate Norton Setup on your windows and Mac device still facing problem call (toll free) +1-844-546-5500 or chat our technical experts they will help you.Norton setup
http://nortonnortoncom.com

19. Get all Steps for installation and download, activate MS office setup. Enter your Office product key on www.office.com/setup.
http://office-comofficecom.com

20. Office.com/setup - Office Setup can be downloaded and installed on your devices by visiting the link office.com/setup online. Microsoft Office suite includes a number of world-leading applications like MS Word, MS Excel, and many others.

21. Office.com/setup- download,Install and activate office setup with product key at office setup.

22. Norton.com/setup offers its clients two different ways to buy any of its items. These two different ways are online by means of Norton.com/setup and disconnected through an approved retail location.
http://nortoncomusa.com/

23. Office.com/setup makes no structures or guarantees that the Office setup Website will be open on a, hair-raising, eating up, secure or mess up free begin.
http://officecomoffice-office.com/

24. Office.com/setup has no relationship with any of these outsider affiliations aside from if such relationship is unequivocally showed up. Office setup permitted use and specific ensures related with the thing, mechanical gathering, and peripherals, you should contact the key removed.
http://office-comoffice.com/index-2.html

25. Office.com/setup makes no diagrams or guarantees that the Office setup Website will be open on a, solid, determined, secure or goof free present. Any Content got to, downloaded or by and large got on or using the website is utilized at your own particular watchfulness.
http://offiicecom-setup.com/

26. Are you seeking for assignment help in the USA? I am an assignment writer in students assignment help.com and I have more than 10 years of experience in assignment writing so if you need help visit our website and get online help.

27. Ireland Assignment Help provides the best Dissertation help for all students. Our professional expert writers provide academic assistance services to all students. Students can get help from our online assignment writers 24*7.