Multiwire linear equalizer for vector signaling code receiver
Continuous-time linear equalization of received signals on multiple wire channels while maintaining accurate common mode signal values. Multiwire group signaling using vector signaling codes simultaneously transmits encoded values on multiple wires, requiring multiple receive signals to be sampled simultaneously to retrieve the full transmitted code word. By misaligning transitions on multiple wires, skew introduces a transient common mode signal component that is preserved by using frequency-selective common mode feedback at the receiver to obtain accurate sampling results.
1. An apparatus comprising:
a plurality of transistors, each transistor of the plurality of transistors having (i) an input node connected to a respective wire of a multi-wire communication medium, (ii) a drain-connected load impedance, (iii) a transistor drain signal output node, (iv) a transistor source node and (v) a source-connected current sink;
a plurality of degeneration impedances, each separately connecting respective transistor sources to a common node; and,
a common node capacitance connected to the common node and to effective signal ground, having a frequency-selective coupling to the effective signal ground, and configured to provide common mode negative feedback signals to the plurality of transistors for common mode signals in a first frequency range and to shunt common mode signals to the effective signal ground for common mode signals in a second frequency range.
2. The apparatus of claim 1 , where the load impedance is selected from the group consisting of a resistor, an inductor, and a resistor-inductor network.
3. The apparatus of claim 1 , where the degeneration impedance comprises a parallel resistor and capacitor network.
4. The apparatus of claim 3 wherein the degeneration impedance is frequency selective to provide increased amplifier gain for frequencies above a predetermined peaking frequency.
5. The apparatus of claim 1 , where signals on the transistor drain signal output nodes are sampled a time instants to offset differential input delays.
6. The apparatus of claim 1 , where the wires of the multi-wire communication medium convey signals of a balanced vector signaling code.
7. An apparatus comprising:
a multi-wire communications medium comprising a plurality of wires;
a multi-input amplifier having a plurality of amplifier branches, each amplifier branch of the plurality of amplifier branches connected a respective wire of the multi-wire communications medium;
a frequency-selective common mode impedance electrically interconnecting the plurality of amplifier branches in a differential amplifier configuration for signals below a first cutoff frequency and electrically isolating the plurality of amplifier branches in a non-differential amplifier configuration for signals above a first cutoff frequency.
8. The apparatus of claim 7 wherein the first cutoff frequency is configured to provide greater than 6 dB of attenuation of frequencies approximately equal to 1/(2Tskew), where Tskew is a maximum timing offset between signal pulses applied to respective transistor amplifiers.
9. The apparatus of claim 7 wherein each of the plurality of source-connected degeneration impedances is frequency selective and provides reduced impedance magnitude at higher frequencies to provide high frequency signal peaking.
10. A method comprising:
receiving signals from a plurality of wires of a multi-wire communications medium at a multi-input amplifier having a plurality of amplifier branches;
conveying low-frequency common mode signals as common mode negative feedback to the plurality of amplifier branches via a frequency-selective common mode impedance; and,
shunting common mode signals above the cutoff frequency to effective signal ground via the frequency-selective common mode impedance.
11. The method of claim 10 further comprising:
increasing an amplifier gain of each amplifier branch as a function of frequency by decreasing a magnitude of a high-frequency component of amplifier branch degeneration impedances via a capacitor in the degeneration impedance.
12. The method of claim 11 , where the degeneration impedance comprises a parallel resistor and capacitor network.
13. The method of claim 10 further comprising sampling output signals of the amplifier branches using time instants that offset differential input delays.
14. The method of claim 10 further comprising applying outputs of the amplifier branches to a plurality of comparators.