Input current-tolerant amplifier input stage for MEMS sensors and other devices
An interface circuit comprises a signal path including a front-end charge amplifier coupling an input of the interface circuit to an output of the interface circuit, and a DC control loop separate from the signal path. In some implementations, the interface circuit is part of a MEMS sensor that includes a MEMS transducer having an output coupled to the input of the interface circuit. The interface circuit can, in some cases, allow faster settling of the circuit to its steady-state operating point.
1. An interface circuit comprising:
a signal path including a front-end charge amplifier coupling an input of the interface circuit to an output of the interface circuit; and
a DC control loop separate from the signal path, wherein the front-end charge amplifier is not part of the DC control loop.
2. The interface circuit of claim 1 wherein the front-end charge amplifier has an input operating point and a low frequency gain, and wherein the DC control loop includes a second amplifier having an input operating point and a low frequency gain that are the same as for the front-end charge amplifier.
3. The interface circuit of claim 2 wherein the DC control loop includes an integrator or low-pass filter, either
wherein first and second high resistance circuit elements are coupled between an input of the front-end charge amplifier and an input of the second amplifier; or
wherein the integrator or low-pass filter is coupled to an output of the second amplifier, and wherein a first high resistance circuit element is coupled between an output of the integrator or low-pass filter and an input of the second amplifier, and a second high resistance circuit element is coupled between the output of the integrator or low-pass filter and an input of the front-end charge amplifier.
4. The interface circuit of claim 3 wherein the interface circuit further includes a capacitor coupled between the output of the integrator or low-pass filter and the input of the second amplifier.
5. The interface circuit of claim 3 wherein at least one of the first or second high resistance circuit elements includes a pair of anti-parallel diodes.
6. The interface circuit of claim 3 further including:
a first switch in parallel to the first high resistance circuit element; and
a second switch in parallel to the second high resistance circuit element.
7. The interface circuit of claim 6 wherein the first and second switches are configured to be closed
during a start-up operation; and/or
after occurrence of an overload condition.
8. The interface circuit of claim 4 further including:
a first switch in parallel to the first high resistance circuit element; and
a second switch in parallel to the capacitor.
9. A MEMS sensor comprising:
an interface circuit having an input and an output;
a MEMS transducer having an output coupled to the input of the interface circuit, wherein the interface circuit includes:
a signal path including a front-end charge amplifier coupling the input of the interface circuit to the output of the interface circuit; and
a DC control loop separate from the signal path, wherein the front-end charge amplifier is not part of the DC control loop.
10. The MEMS sensor of claim 9 wherein the front-end charge amplifier has an input operating point and a low frequency gain, and wherein the DC control loop includes a second amplifier having an input operating point and a low frequency gain that are the same as for the front-end charge amplifier.
11. The MEMS sensor of claim 10 wherein the DC control loop includes an integrator or low-pass filter, either:
wherein first and second high resistance circuit elements are coupled between an input of the front-end charge amplifier and an input of the second amplifier; or
wherein the integrator or low-pass filter is coupled to an output of the second amplifier, and wherein a first high resistance circuit element is coupled between an output of the integrator or low-pass filter and an input of the second amplifier, and a second high resistance circuit element is coupled between the output of the integrator or low-pass filter and an input of the front-end charge amplifier.
12. The MEMS sensor of claim 11 wherein the interface circuit further includes a capacitor coupled between the output of the integrator or low-pass filter and the input of the second amplifier.
13. The MEMS sensor of claim 11 wherein at least one of the first or second high resistance circuit elements includes a pair of anti-parallel diodes.
14. The MEMS sensor of claim 11 further including:
a first switch in parallel to the first high resistance circuit element; and
a second switch in parallel to the second high resistance circuit element.
15. The MEMS sensor of claim 14 wherein the first and second switches are configured to be closed:
during a start-up operation; and/or
after occurrence of an overload condition.
16. The MEMS sensor of claim 12 further including:
a first switch in parallel to the first high resistance circuit element; and
a second switch in parallel to the capacitor.
17. The MEMS sensor of claim 9 wherein the interface circuit has differential inputs and outputs.
18. A MEMS sensor comprising:
an interface circuit having an input and an output;
a MEMS transducer having an output coupled to the input of the interface circuit, wherein the interface circuit includes:
a signal path including a front-end charge amplifier coupling the input of the interface circuit to the output of the interface circuit; and
a DC control loop including:
a second amplifier having an input and an output;
an integrator or low-pass filter having an input coupled to the output of the second amplifier;
a capacitor coupled between an output of the integrator or low-pass filter and the input of the second amplifier;
the interface circuit further including a first high resistive circuit element and a second high resistive circuit element, wherein at least one of the high resistive circuit elements is coupled between the output of the integrator or low-pass filter and the input of the front-end charge amplifier.
19. The MEMS sensor of claim 18 wherein the MEMS transducer is configured as a MEMS microphone.