Before the birth of EMI hardened amplifiers, system designers implemented their own filtering scheme. Some worked and others not so well.
A common mishap is to insert a capacitor across the inputs of the amplifier. This approach can cause serious stability issues and usually requires some sort of compensation.
Over the past few years (at least 3), every precision amplifier released by TI 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 because the rejection depends on where the cut off frequency is set as with respect to the amplifier bandwidth.
To avoid introducing a phase lag, the IC designer generally picks a cut off that is at least 10x that of the unity gain of the op amp. The order of the filter also determines how much rejection (attenuation) the op amp provides.
As an example 1MHz with a first order filter at 10MHz will reject 40dB at 1GHz. However, a 10MHz 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 and 30dB of rejection (1GHz). So we have 316mV at the output of the op amp [(100mV/31.6)*100]. Let’s now assume the output is fed to a 12 ADC with a 5V FSR.
Let’s compute the loss of counts caused the injected signal (EMI):
5V/(2^12)=1.22mV, now diving the output of the op amp by 1.22mV (316mV), we see that we lose nearly 260 counts.
Using the OPA172 reduces the count loss to roughly 8. Using 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 have the peace of mind knowing 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).