High input impedance low power ADC
An electronic system is disclosed. The electronic system includes an input transistor having a source configured to receive an analog input signal; a filter having an input coupled to an output node of the input transistor; an ADC having an input coupled to an output of the filter and an output configured to provide a digital signal representative of the analog input signal; and a DAC having an input coupled to the output of the ADC and an output configured to provide a current to the source of the input transistor.
1 . An electronic system, comprising:
an input transistor having a gate configured to receive an analog input signal;
a filter having an input coupled to an output node of the input transistor;
an ADC having an input coupled to an output of the filter and an output configured to provide a digital signal representative of the analog input signal; and
a DAC having an input coupled to the output of the ADC and an output configured to provide a current to a source of the input transistor.
2 . The electronic system of claim 1 , wherein the input transistor, the ADC, and the DAC form a noise shaping loop.
3 . The electronic system of claim 2 , further comprising:
a first current source electrically connected to the source of the input transistor; and
a second current source electrically connected to a drain of the input transistor.
4 . The electronic system of claim 3 , further comprising a resistor connected to the source of the input transistor.
5 . The electronic system of claim 1 , wherein a signal gain between the gate of the input transistor and an input to the filter is greater than 0 dB.
6 . The electronic system of claim 1 , wherein the filter comprises an input coupled to a reference current source.
7 . The electronic system of claim 6 , wherein the filter comprises an input coupled to a reference voltage source.
8 . An electronic system, comprising:
an input transistor having a gate configured to receive an analog input signal;
a filter having an input coupled to an output node of the input transistor;
an oscillator having a frequency control input coupled to an output of the filter; and
a frequency to current converter having an input coupled to an output of the oscillator and an output configured to provide a current to a source of the input transistor.
9 . The electronic system of claim 8 , further comprising a frequency to digital converter having an input coupled to the output of the oscillator and an output configured to provide a digital representation of the analog input signal.
10 . The electronic system of claim 9 , wherein the input transistor, the filter, the oscillator, and the frequency to current converter form a noise shaping loop.
11 . The electronic system of claim 8 , further comprising:
a first current source electrically connected to the source of the input transistor; and
a second current source electrically connected to a drain of the input transistor.
12 . The electronic system of claim 8 , wherein the filter comprises an input coupled to a reference current source.
13 . The electronic system of claim 8 , wherein the filter comprises an input coupled to a reference voltage source.
14 . The electronic system of claim 8 , further comprising a resistor connected to the source of the input transistor.
15 . The electronic system of claim 8 , wherein a signal gain between the gate of the input transistor and an input to the filter is greater than 0 dB.
16 . The electronic system of claim 8 , wherein the frequency to current converter comprises a frequency dependent resistor.
17 . A method of using an electronic system, the method comprising:
with a source follower circuit, generating an output signal based on a difference between an input signal and a feedback signal, wherein the source follower circuit has a gain greater than one;
generating a filtered difference signal based on the output signal and a reference signal; and
generating a digital signal based on the filtered difference signal, wherein the digital signal corresponds with the input signal.
18 . The method of claim 17 , further comprising generating the feedback signal based on the filtered difference signal.
19 . The method of claim 17 , further comprising:
generating a frequency signal based on the filtered difference signal; and
generating the digital signal based on the frequency signal.
20 . The method of claim 17 , further comprising:
generating a frequency signal based on the filtered difference signal; and
generating the feedback signal based on the frequency signal.