A common requirement when capturing signals from sensors such as Electro Magnetic Flow Meters (EM Flow) in Field transmitters or electrodes in biopotential applications, the signal of interest is usually sitting on much larger dc offsets. To make it easier to extract the relevant information from these sensors the precision low power signal chain team have recently published two different KWIK (Know-how With Integrated Knowledge) AC coupled signal conditioning circuits. In both circuits, the aim is to remove the DC offsets while amplifying the AC signals, each with their own benefits and tradeoffs.

KWIK 1: “Amplifying AC signals with large DC offsets for Low Power Designs” covers a typical AC coupled signal chain which includes a low gain instrumentation amplifier, followed by a high pass filter and an additional gain stage. The gain setting depends greatly on the input signal and offset amplitude, as well as the supply voltage to avoid saturation. This circuit allows for more flexibility since all instrumentation amplifier architectures can be used, which allows for lower power and smaller footprint choices such as a Wafer Level Chip Scale Package (WLCSP). The high pass filter also has a settling time dependent on the resistor and capacitor value, while KWIK 2 adds a dependency on the gain as well (slower time constants with increasing gains).

KWIK 2: “Amplifying AC signals in the presence of large DC offsets using an Indirect Current-Mode Instrumentation Amplifier” uses a different architecture compared to those found in typical AC coupled circuits. It includes an Indirect Current Mode Instrumentation Amplifier, providing the gain and the AC coupled all in one stage by incorporating an integrator circuit in the feedback loop. This circuit allows removal of the DC offset at the very input prior to applying gain. Moreover, thanks to this topology, a substantial gain can be implemented leading to lower Referred to Input (RTI) Noise.

On the following table, a summary of tradeoffs comparing KWIK 1 and KWIK 2 can be found.

Table 1. Summary of KWIK 1 vs. KWIK 2 features 

Considerations

KWIK 1

KWIK 2

COMMENTS

Noise

 

White check mark

KWIK 2 can improve RTI noise by having higher front-end gain

DC offset

 

White check mark

KWIK 2 allows for a much larger input DC offset range

Settling Time

White check mark

 

KWIK1’s settling time depends on the high-pass filter components while KWIK2 adds a gain dependency (slower time constants with increasing gains)

Flexibility

White check mark

 

There are fewer component choices for Indirect Current Mode Instrumentation Amplifiers

Component Count

White check mark

White check mark

Approximately the same.

Size

White check mark

 

There are more WLCSP offerings available for the KWIK1 architecture.

Power Consumptions

White check mark

White check mark

Both solutions have low power components to choose from.

References

KWIK 1: “Amplifying AC signals with large DC offsets for Low Power Designs "

KWIK 2: “Amplifying AC signals in the presence of large DC offsets using an Indirect Current-Mode Instrumentation Amplifier

EZ Blog: Precision Low Power Signal Chains: A Unique AC Coupled Solution with Configurability 

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