Figure 54. A1 amplification + intrinsic offset

Figure 55. Unchopped output diff. voltage + filter

The DC offset and low-frequency noise introduced by A1 are converted to the residual square ripple seen on

Figure 55. Due to many non-idealities of the notch filter and some complications in the reconstruction of the

signal, the output of the TSB181, TSB182 operational amplifiers can show some of the switching characteristic of

the chopping stages, including output ripple and converging steps, especially in some configurations where a fast

transient is applied at the input or when driven out of saturation. These non-analog behaviors are drawbacks

inherent to the chopper architecture, hence special efforts were made to reduce them as much as possible, such

as lowering the output ripple down to the TSB181, TSB182 operational amplifiers' output noise level.

Figure 56. Normalized cardinal sine

Figure 57. Output ripple and notch sampler

The ripple is in fact not at first visible in terms of steps but is more likely a triangular waveform, as the first

amplifying stage is driving its output capacitor in current. The ripple is then reduced by a notch filter, a sample-

and-hold system taking a picture of the voltage each period of the triangle wave. If tuned wisely, this notch filter

can dramatically reduce the output ripple down to a very low level which is acceptable compared to the achieved

Vio performance. This is made possible thanks to the frequency characteristics of the notch filter, totally rejecting

some frequencies (in theory), as shown in Figure 56. The filter is made so that the chopping frequency and its

harmonics are canceled out, thus eliminating the ripple, and removing the remaining errors that were at or close

to the chopping frequency (1/f noise).

TSB181, TSB182

Application information

DS14357 - Rev 5

page 20/36