Hi. My name is Mike, and I’m an application support engineer with Agilent. This short
video will describe the DC and AC common mode specifications for our DMMs.
Let’s start with a very typical situation where we’re inputting a voltage to our DMM
and trying to make a measurement. I’m going to distinguish between the normal mode voltage,
and this is basically the input voltage that we’re trying to measure. We’re connecting
it to the high and the low terminals of our DMM, and typically there’s some small
lead resistance or RL associated with making that connection.
Contrast the normal mode voltage with the common mode voltage. This is basically the
floating voltage. This is the voltage between earth ground and our input voltage. Most DMMs
allow an input to be floated above or below earth ground, somewhere between 300 and 500
volts, and still make an accurate normal mode voltage measurement. However, there are some
small errors associated with this Vf or the common mode voltage.
So what is the DC common mode rejection ratio? This is the ability of the DMM to reject this
Vf, and not have it impact the measurement between the high and the low terminal, because
see, the arrow shows the current flow from Vf. It will flow through RL on the low terminal,
and then through Ri returning to earth ground.
The common mode rejection ratio for the 34410A and 3411A is 140 dB, and as the footnote says,
“This assumes a 1 k-ohm unbalance in the low lead”. This is a worst-case situation.
Typically you don’t have a k-ohm lead resistance, but with the k-ohm, and a small current flowing
through it due to this floating voltage, we would get an error term in between the
high and low terminals. So let’s determine how much that would be.
Basically it’s going to be a resistive divider between RL and Ri. In this case we’re doing
a DC common mode rejection ratio so the capacitor is not involved in this calculation.
So let’s go through that calculation briefly and see what error term we get.
The common mode error voltage, which is the voltage drop across that lead resistance in
the low lead, due to our floating voltage, can be calculated with a simple resistive
divider with the lead resistance of 1 k-ohm in the input resistance of ten giga-ohms.
So if you plug in those values into our equation, we see that the common mode rejection ratio
is 9.999999 times ten to the minus eighth, very small number. It’s often useful to
represent that in log scale so if we convert to log scale, we end up with minus 140 dB
or we can also state that as a 140 dB of rejection or reduction. So that’s a significant amount
of reduction of the floating voltage, and it is very, very negligible when measuring
our normal mode or our input voltage.
We have a very, very similar situation with AC. Remember there was a capacitance involved.
So the impedance between the low terminal and earth ground is not as high in the AC
situation. The specification says that the AC common mode rejection is 70 dB, and the
footnote points out that there is still a 1 k-ohm imbalance in the low lead resistance
where we have an AC signal of 60 hertz or less. Again, the capacitor comes into play,
the impedance between low and earth ground is much less and, therefore, our AC common
mode rejection is less, at 70 dB.
This concludes this short video describing the DC and AC common mode rejection
specifications for our DMMs.