Architecture for DC bias canceling and gain recovery for hard disk drive fly height detection
According to an embodiment, a system for processing signals from a resistive sensor that monitors spacing between a disk drive head and disk surface includes a bias-canceling digital-to-analog converter and a translinear cell. The bias-canceling digital-to-analog converter generates a cancelation current equal to a bias current across the resistive sensor. The translinear cell receives the cancelation current and a composite current comprising the bias current and a sensor signal current, where the sensor signal current corresponds to variations in thermal conductivity as the disk drive head approaches the disk surface. The translinear cell generates differential output currents proportional to a sensing voltage while eliminating dependence on resistance value of the resistive sensor. A front-end circuit combines these differential output currents with signals coupled directly from sensor nodes at frequencies above a bias loop bandwidth.
1 . A system for processing signals from a resistive sensor that monitors spacing between a disk drive head and a disk surface, the system comprising:
a bias-canceling digital-to-analog converter configured to generate a cancelation current, the cancelation current equal to a bias current across the resistive sensor; and
a translinear cell configured to:
receive the cancelation current and a composite current comprising the bias current and a sensor signal current, wherein the sensor signal current corresponds to variations in thermal conductivity as the disk drive head approaches the disk surface, and
generate an output current proportional to a sensing voltage across the resistive sensor to eliminate dependence on resistance value of the resistive sensor.
2 . The system of claim 1 , further comprising a front-end circuit configured to receive differential output currents from the translinear cell and combine the differential output currents with signals coupled from sensor nodes at frequencies above a bias loop bandwidth.
3 . The system of claim 2 , wherein the cancelation current equals the bias current in response to differential outputs of the front-end circuit being balanced.
4 . The system of claim 2 , further comprising:
a low-pass filter configured to receive differential output voltages of the front-end circuit; and
a gain stage coupled to the low-pass filter, wherein the gain stage is configured to amplify filtered signals while maintaining linear operation.
5 . The system of claim 1 , further comprising a biasing operational amplifier configured to establish a bias voltage across the resistive sensor through a voltage feedback loop.
6 . The system of claim 1 , further comprising a current source coupled between the bias-canceling digital-to-analog converter and the translinear cell.
7 . The system of claim 1 , wherein the output current maintains constant gain independent of temperature variations of the resistive sensor.
8 . A circuit for monitoring spacing between a disk drive head and a disk surface using a resistive sensor that detects variations in thermal conductivity as the disk drive head approaches the disk surface, the circuit comprising:
a first transistor configured to receive a cancelation current, wherein the cancelation current is equal to a bias current flowing through the resistive sensor;
a second transistor and a third transistor coupled to a first output node and a second output node, respectively; and
a fourth transistor receiving a composite current comprising the bias current and a sensor signal current from the resistive sensor,
wherein the first transistor, the second transistor, the third transistor, and the fourth transistor are arranged in a translinear configuration to generate differential output currents proportional to a sensing voltage while eliminating dependence on resistance value of the resistive sensor.
9 . The circuit of claim 8 , wherein the differential output currents are provided to a front-end circuit that combines the differential output currents with signals coupled directly from sensor nodes at frequencies above a bias loop bandwidth.
10 . The circuit of claim 9 , wherein the cancelation current equals the bias current in response to differential outputs of the front-end circuit being balanced.
11 . The circuit of claim 8 , wherein the circuit is coupled to a bias-canceling digital-to-analog converter through a current source, the bias-canceling digital-to-analog converter configured to generate the cancelation current.
12 . The circuit of claim 8 , wherein the circuit is coupled to a biasing operational amplifier configured to establish a bias voltage across the resistive sensor through a voltage feedback loop.
13 . The circuit of claim 8 , wherein base-emitter voltages of the first transistor, the second transistor, the third transistor, and the fourth transistor satisfy Kirchoff's voltage law around a closed loop to maintain constant gain independent of temperature variations.
14 . The circuit of claim 8 , further comprising a current source configured to generate a reference current proportional to a ratio of a bias voltage to a reference resistance.
15 . A method for monitoring spacing between a disk drive head and a disk surface using a resistive sensor, the method comprising:
establishing a bias voltage across the resistive sensor through a voltage feedback loop to generate a bias current;
generating a cancelation current equal to the bias current;
receiving, at a translinear cell, the cancelation current and a composite current comprising the bias current and a sensor signal current, wherein the sensor signal current corresponds to variations in thermal conductivity as the disk drive head approaches the disk surface;
generating, by the translinear cell, differential output currents proportional to a sensing voltage across the resistive sensor while eliminating dependence on resistance value of the resistive sensor; and
processing the differential output currents to determine the spacing between the disk drive head and the disk surface.
16 . The method of claim 15 , wherein processing the differential output currents comprises combining, in a front-end circuit, the differential output currents with signals coupled directly from sensor nodes at frequencies above a bias loop bandwidth.
17 . The method of claim 16 , further comprising adjusting the cancelation current until differential outputs of the front-end circuit are balanced.
18 . The method of claim 16 , further comprising:
filtering the differential outputs of the front-end circuit using a low-pass filter; and
amplifying the filtered signals using a gain stage while maintaining linear operation.
19 . The method of claim 15 , wherein generating the differential output currents comprises processing the currents through transistors arranged in a translinear configuration that maintains constant gain independent of temperature variations.
20 . The method of claim 15 , further comprising generating a reference current proportional to a ratio of the bias voltage to a reference resistance.