Temperature and drift compensation in magnetoresistive sensors
Double modulation of a magnetoresistive sensor entails modulating both an excitation (e.g., voltage or current) applied to the sensor and a tickling magnetic field applied to the sensor. The excitation and magnetic field are modulated at different frequencies f c and f f , respectively. As a result of the double modulation, the sensor output spectrum includes a carrier tone (CT) at frequency f c and side tones (STs) at frequencies f c ±f f . A baseline relation between CT amplitude and ST amplitude is determined (e.g., by measuring CT and ST amplitude while drift occurs in the absence of a sample). During sensor operation, raw ST measurements are corrected using corresponding raw CT measurements to provide corrected ST measurements as the sensor output.
1. A magnetoresistive sensor apparatus comprising:
a) a magnetoresistive sensor;
b) a magnetic field source that provides a tickling magnetic field having a modulation frequency of f f , wherein said magnetoresistive sensor is disposed in said tickling magnetic field;
c) an electrical source configured to provide an excitation to said magnetoresistive sensor having a modulation frequency of f c , wherein f f and f c , are distinct, and wherein an output spectrum of said sensor includes a carrier tone (CT) at frequency f c and side tones (STs) at frequencies f c ±f f , wherein the carrier tone has a CT amplitude and the side tones have an ST amplitude; and
d) a processor configured to determine a baseline relation between the CT amplitude and the ST amplitude in said output spectrum;
wherein operation of said apparatus includes measuring corresponding CT and ST amplitudes when a sample is present to provide an input CT measurement and an input ST measurement;
wherein said apparatus is configured to correct the input ST measurement using said baseline relation and the input CT measurement to provide a corrected ST measurement as an output.
2. The apparatus of claim 1 , wherein said corrected ST measurement is provided in real time during acquisition of said input CT measurement and said input ST measurement.
3. The apparatus of claim 1 , wherein said corrected ST measurement is provided by post-processing after acquisition of said input CT measurement and said input ST measurement-a.
4. The apparatus of claim 1 , wherein said electrical source is a voltage source or a current source.
5. A method for correcting the output of a magnetoresistive sensor, the method comprising:
a) providing a magnetoresistive sensor;
b) disposing said magnetoresistive sensor in a tickling magnetic field having a modulation frequency of f f ;
c) providing an excitation to said magnetoresistive sensor having a modulation frequency of f c , wherein f f and f c are distinct, and wherein an output spectrum of said sensor includes a carrier tone (CT) at frequency f c and side tones (STs) at frequencies f c ±f f , wherein the carrier tone has a CT amplitude and the side tones have an ST amplitude;
d) determining a baseline relation between the CT amplitude and the ST amplitude in said output spectrum;
e) measuring corresponding CT and ST amplitudes when a sample is present to provide an input CT measurement and an input ST measurement;
f) correcting said input ST measurement using said baseline relation and said input CT measurement to provide a corrected ST measurement; and
g) providing said corrected ST measurement as an output.
6. The method of claim 5 , further comprising repeating said steps (e), (f), and (g) sequentially in order to provide real time correction for multiple ST measurements.
7. The method of claim 5 , further comprising repeating said step (e) to provide a set of input (CT, ST) data points, wherein said corrected ST measurement for each of said data points is provided by post-processing said set of input (CT, ST) data points.
8. The method of claim 5 , wherein said determining a baseline relation between CT amplitude and ST amplitude comprises collecting ST vs. CT data as the temperature of said magnetoresistive sensor varies.
9. The method of claim 5 , wherein said determining a baseline relation between CT amplitude and ST amplitude comprises collecting ST vs. CT data as an input to said magnetoresistive sensor varies.
10. The method of claim 5 , wherein said determining a baseline relation between CT amplitude and ST amplitude comprises collecting ST vs. CT data as an environmental parameter of said magnetoresistive sensor varies.
11. The method of claim 5 , wherein said determining a baseline relation between CT amplitude and ST amplitude comprises collecting ST vs. CT data followed by determining a curve fit relating baseline ST amplitude to baseline CT amplitude.
12. The method of claim 11 , wherein said curve fit is a linear fit.
13. The method of claim 5 , wherein said sample comprises paramagnetic or superparamagnetic nano-particles.
14. The method of claim 13 , wherein said nano-particles are labels on biological molecules.
15. The method of claim 5 , wherein said excitation is a voltage or current applied to said magnetoresistive sensor.