Isolation communications channel using direct demodulation and data-edge encoding

1. An apparatus for communicating across an isolation barrier, the apparatus comprising:

a differential pair of input terminals;

a bandpass filter circuit configured to receive a received signal on the differential pair of input terminals and to provide a received differential signal on a differential pair of nodes;

a demodulator directly coupled to the bandpass filter circuit and configured to directly demodulate the received differential signal on the differential pair of nodes to provide a demodulated received signal; and

a transmitter path including an oscillator circuit configured to provide an oscillating signal to a differential pair of output terminals in response to a control signal, wherein the bandpass filter circuit includes a first inductor and a first capacitor matched to a second inductor and a second capacitor of the oscillator circuit.

2. The apparatus of claim 1 wherein the bandpass filter circuit is configured to amplify a first frequency band of the received signal and to attenuate a second frequency band of the received signal, a carrier signal of the received signal being in the first frequency band and common-mode transient interference of the received signal being in the second frequency band.

3. The apparatus of claim 1 wherein the demodulator includes a differential pair of transistors, each transistor of the differential pair of transistors having a corresponding source terminal coupled to a corresponding node of the differential pair of nodes.

4. The apparatus of claim 1 , further comprising a digital circuit configured to provide a received digital signal based on the demodulated received signal, the digital circuit being configured to toggle the received digital signal to a first logic value in response to a pulse of the demodulated received signal having a first width and configured to toggle the received digital signal to a second logic value in response to a second pulse of the demodulated received signal having a second width.

5. The apparatus of claim 1 ,

further comprising:

the oscillator circuit coupled to the differential pair of output terminals and configured to transmit a signal using the differential pair of output terminals according to a transmit data signal received on a center tap of the second inductor of the oscillator circuit; and

a control circuit configured to generate the transmit data signal having a first modulated pulse with a first pulse width in response to a first transition of an input data signal and to generate a second modulated pulse having a second pulse width in response to a second transition of the input data signal, the first pulse width being greater than the second pulse width.

6. An apparatus for communicating across an isolation barrier, the apparatus comprising:

a differential pair of input terminals;

a bandpass filter circuit configured to receive a received signal on the differential pair of input terminals and to provide a received differential signal on a differential pair of nodes;

a demodulator directly coupled to the bandpass filter circuit and configured to directly demodulate the received differential signal on the differential pair of nodes to provide a demodulated received signal, the demodulator including a differential pair of transistors, each transistor of the differential pair of transistors having a corresponding source terminal coupled to a corresponding node of the differential pair of nodes; and a reference transistor having a first size greater than a sum of second sizes of transistors of the differential pair of transistors.

7. The apparatus of claim 6 further comprising a transmitter path including an oscillator circuit configured to provide an oscillating signal to a differential pair of output terminals in response to a control signal, the bandpass filter circuit including a first inductor and a first capacitor matched to a second inductor and a second capacitor of the oscillator circuit.

8. The apparatus of claim 6 wherein an offset threshold of the demodulator is determined by a ratio of the first size of the reference transistor and a second size of each transistor of the differential pair of transistors.

9. The apparatus of claim 6 wherein the demodulator further includes:

a first resistor coupled between a common node coupled to a first source terminal of the reference transistor and a second source terminal of a first transistor of the differential pair of transistors; and

a second resistor coupled between the common node and a third source terminal of a second transistor of the differential pair of transistors, the first resistor and the second resistor having the same resistance.

10. The apparatus of claim 6 wherein the demodulator further includes a bias transistor having a first gate terminal coupled to second gate terminals of the differential pair of transistors and a third gate terminal of the reference transistor.

11. The apparatus of claim 6 wherein the bandpass filter circuit is configured to amplify a first frequency band of the received signal and to attenuate a second frequency band of the received signal, a carrier signal of the received signal being in the first frequency band and common-mode transient interference of the received signal being in the second frequency band.

12. The apparatus of claim 6 , further comprising a digital circuit configured to provide a received digital signal based on the demodulated received signal, the digital circuit being configured to toggle the received digital signal to a first logic value in response to a pulse of the demodulated received signal having a first width and configured to toggle the received digital signal to a second logic value in response to a second pulse of the demodulated received signal having a second width.

13. The apparatus of claim 6 , further comprising:

an oscillator circuit coupled to a differential pair of output terminals and configured to transmit a signal using the differential pair of output terminals according to a transmit data signal received on a center tap of an inductor of the oscillator circuit; and

a control circuit configured to generate the transmit data signal having a first modulated pulse with a first pulse width in response to a first transition of an input data signal and to generate a second modulated pulse having a second pulse width in response to a second transition of the input data signal, the first pulse width being greater than the second pulse width.

14. A method for communicating across an isolation barrier, the method comprising:

bandpass filtering a received signal on a differential pair of input terminals to provide a received differential signal on a differential pair of nodes;

directly demodulating the received differential signal on the differential pair of nodes to provide a demodulated received signal;

selectively configuring a first instantiation of an LC circuit on a first integrated circuit die as an LC oscillator circuit; and

selectively configuring a second instantiation of the LC circuit on a second integrated circuit die as a bandpass filter circuit.

15. The method of claim 14 wherein directly demodulating comprises:

biasing a differential pair of transistors;

receiving the received differential signal on first source terminals of the differential pair of transistors; and

generating an indication of a difference between a first voltage on drain terminals of the differential pair of transistors to a second voltage on a drain of a reference transistor.

16. The method of claim 14 , further comprising:

decoding a first value of a digital signal based on first pulse having a first pulse width; and

decoding a second value of the digital signal based on a second pulse having a second pulse width smaller than the first pulse width.

17. The method of claim 14 wherein the bandpass filtering amplifies a first frequency band of the received signal and attenuates a second frequency band of the received signal, a carrier signal of the received signal being in the first frequency band and common-mode transient interference of the received signal being in the second frequency band.

18. The method of claim 14 , further comprising transmitting across the isolation barrier, a first modulated pulse in response to a first transition of a data signal and a second modulated pulse in response to a next transition of the data signal, the first modulated pulse having a first pulse width and the second modulated pulse having a second pulse width different from the first pulse width.