0
\$\begingroup\$

From the reading of This datasheet, I can't figure out what is the maximum load I can put at the output. To make my question more precise, suppose I need a gain of 10 (for DC measurements), for an input voltage between 0 and 1V (at very high impedance, that is why I want to use this amplifier (in my stock) that has only a few nanoAmp of input bias current), and assume the supply of the amplifier is generously above 10V.

question 1: What is the maximum load I can put at the output ?

question 2: In p. 15 they say the maximum input current is 6mA (or 60mA for few hours). I thought amplifiers are not supposed to draw any input current, except, of course, the input bias and input offset currents. Can someone explain ?

\$\endgroup\$
  • 1
    \$\begingroup\$ FYI, IIRC AD8221 is a (relatively) modern replacement for AD620 that matches or improves performance nearly line for line with a lower price. \$\endgroup\$ – The Photon May 10 '18 at 16:25
  • \$\begingroup\$ What effective number of bits (ENOB) do you need? Or what linearity must you have? And what about zero-point and full-scale drift? \$\endgroup\$ – analogsystemsrf May 10 '18 at 17:11
  • \$\begingroup\$ Yes, I should also address these questions for my application. But I think that regarding these questions, I can understand the datasheet by myself. \$\endgroup\$ – MikeTeX May 10 '18 at 17:33
4
\$\begingroup\$

Pretty much all of the specifications are only guaranteed with a load of 2 kohms:

enter image description here

The ones where it's "otherwise noted" (for example, nonlinearity as seen in the image, and output voltage swing) generally specify a weaker load of 10 kohms, not a stronger load.

There's also this typical performance curve that shows the output drive capability falls off pretty quickly for load resistance below 2 kohm:

enter image description here

I thought amplifiers are not supposed to draw any input current, except, of course, the input bias and input offset currents. Can someone explain ?

Under normal operation, the inputs will draw very little current.

But, for example, if you drive the input voltages above or below the power supply rails, they might draw considerable current through the ESD protection diodes. You would not expect the amplifier to perform normally (give the desired gain, etc) under these conditions.

One situation where this could happen is if the driving device is powered up before the AD620 power rails are.

\$\endgroup\$
  • \$\begingroup\$ OK for question 2. Regarding question 1 you say: "The ones where it's "otherwise noted" (for example, nonlinearity as seen in the image, and output voltage swing) generally specify a weaker load of 10 kohms, not a stronger load." Do you mean that everything will generally be OK if the load is > 2k ? \$\endgroup\$ – MikeTeX May 10 '18 at 16:37
  • \$\begingroup\$ Yes, it's easier to drive a higher resistance load, so the specs at 2 kohms should hold (or improve) with higher load resistance. \$\endgroup\$ – The Photon May 10 '18 at 16:52
  • \$\begingroup\$ @AliChen, edited. \$\endgroup\$ – The Photon May 10 '18 at 18:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.