A low power and low 1/f-noise transconductance for the input stage of a current sense A/D converter

A low power and low noise transconductance for a current sense A/D converter is presented. The equivalent input noise is 2 muV/VHz @ 10 Hz. The supply current is 6.8 muA at a 2.5 V supply for the transconductance core, including the feedback and the input level shifter. The circuit comprises a native threshold NMOS input stage. The design of the PMOS current mirrors connected to the input stage provides one differential output that can be connected to the integration capacitor of a muSigma-modulator, and a second fully differential output is connected to the input resistors. The second output provides the shift of the DC level of the input signal and the feedback path for adjustment and linearization of the transconductance. Low noise circuit design of input stages is discussed and the characteristics of the transconductance are compared with measured data.