Challenge:
Before the birth of electromagnetic interference (EMI) hardened amplifiers, system designer like yourself was left to implement your own filtering schemes. Some of these schemes worked and others just didn't make the cut, leaving a headache for you.
One of the many common mishaps you might have experienced back then was to insert a capacitor across the inputs of the amplifier. This approach can cause serious stability issues and usually requires some sort of compensation. More work for you to implement.
Solution:
Over the past few years (at least 3), every amplifier released by TI (from the precision group) gives customers the peace of mind knowing that these op amps have internal filters which reject any sort of inadvertent RF injected signal into them.
Of course, not all of them reject the same way. The rejection depends on where the cut off frequency is set with respect to the amplifier bandwidth.
To avoid introducing a phase lag, the IC designer generally picks a cut off that is at least a decade beyond the unity gain bandwidth of the op amp. The order of the filter also determines how much rejection (attenuation) the op amp provides.
As an example a 1MHz op amp with a first order filter at 10MHz will reject 40dB at 1GHz. However, a 10MHz device with a cut off at 100MHz has a rejection of only 20dB at 1GHz.
EMI errors can have serious consequences on the system. Suppose a 100mV is injected into an amplifier with gain of 100. Let’s say you’re using an op amp with no EMI filters but which still provides 30dB of rejection (1GHz).
You should expect to see 316mV at the output of the op amp [(100mV/31.6)*100]. Let’s now assume the output is fed to a 12 bit ADC with a 5V FSR.
We can compute the loss of counts caused the injected signal (EMI) as follows:
5V/(2^12)=1.22mV, now dividing the output of the op amp by 1.22mV (316mV/1.22mV), we determine a loss of nearly 260 counts.
Using an op amp like the OPA172 and the same computation, you reduce the count loss to roughly 8. The LMV831, which provides 90dB of rejection, reduces it further to 0.25!
So the next time you’re looking for a precision op amp, make TI your first stop and know that all of our latest amplifiers have integrated EMI….at no additional cost!
Check out this clip in our video library for some additional interesting information on How to avoid electromagnetic interference (EMI).
