Learn about chopper op amps and noise

The input stage of a chopper op amp is shown in figure 1. The amplifier is a relatively conventional transconductance stage with differential input and differential output current. Chopping is accomplished with commutating switches on the input and output that synchronously reverse the polarity. Since both differential input and output are reversed simultaneously, the net effect on the output capacitor, C1, is a constant signal path polarity.

Figure 1: The input stage of a chopper op amp.

The offset voltage of the transconductance stage is inside the input switching network, so its contribution to output is periodically reversed by the output switches. The output current caused by offset voltage causes the voltage on C1 to ramp up and down at an equal rate. Internal logic assures equal up and down ramp times so the average output voltage on C1 is zero. Thus, zero offset!

Early generation choppers provided only modest filtering of the triangular chopping noise, causing them to be branded as wickedly noisy devices, used only when very low offset voltage was crucial. (And this is how big, noisy motorcycles got their name.) Particularly troublesome was that the pre-chopping offset voltage determined the magnitude of the triangle waveform so chopping noise could vary considerably from unit to unit.

Figure 2: In the frequency domain, this creates a sinc(x) or sin(x)/x filter response with nulls that precisely align with the fundamental and all harmonics of the triangle wave.

New generation choppers are dramatically quieter, incorporating a switched-capacitor filter with multiple notches aligned with the chopping frequency and its odd harmonics. This is accomplished by integrating charge on C1 for a full cycle before transferring its charge to the next stage of the op amp. Integrated over a full up-down cycle, its net value is zero—perfectly averaged. In the frequency domain, this creates a sinc(x) or sin(x)/x filter response with nulls that precisely align with the fundamental and all harmonics of the triangle wave (figure 2).

1 • 2