Resistance measurement and current control
A technique for measuring the resistance of a resistive element 4 in the presence of a series diode is provided. By supplying three different currents I 1 , I 2 , I 3 and measuring corresponding voltages V 1 , V 2 , V 3 across the resistive element and diode, the voltages can be combined to at least partially eliminate an error in the measured resistance of the resistive element caused by a voltage drop across the diode. A technique for current control in an array of resistive elements is also described in which a column of resistive elements is provided with two or more current sources switched so that while one current source is providing current to the column line corresponding to a selected resistive element, another current source has its amount of current adjusted.
1. An apparatus comprising:
a resistive element and a diode arranged in series;
current control circuitry configured to switch a current supplied to the resistive element and the diode between a first current I 1 , a second current I 2 and a third current I 3 , where I 1 >I 2 >I 3 ; and
measuring circuitry configured to determine a resistance R of the resistive element in dependence on a first voltage V 1 across the resistive element and the diode when the first current I 1 is supplied, a second voltage V 2 across the resistive element and the diode when the second current I 2 is supplied, and a third voltage V 3 across the resistive element and the diode when the third current I 3 is supplied;
wherein the measuring circuitry is configured to combine the first voltage V 1 , the second voltage V 2 and the third voltage V 3 to at least partially eliminate an error in the determined resistance R of the resistive element caused by a voltage drop across the diode;
wherein the resistive element is one of an array of resistive elements arranged in rows and columns, wherein each resistive element is individually addressable via a corresponding row line and a corresponding column line and is arranged in series with a diode between the corresponding row line and the corresponding column line.
2. The apparatus according to claim 1 , wherein a ratio between the first current I 1 and the second current I 2 is substantially the same as a ratio between the second current I 2 and the third current I 3 .
3. The apparatus according to claim 1 , wherein the measuring circuitry is configured to determine the resistance R of the resistive element proportional to V 1 −2V 2 +V 3 .
4. The apparatus according to claim 1 , wherein the measuring circuitry comprises an analogue-to-digital converter configured to generate a digital value representing the determined resistance R of the resistive element in dependence on the first voltage V 1 , second voltage V 2 , and third voltage V 3 detected during a single sampling period of the analogue-to-digital converter.
5. The apparatus according to claim 1 , wherein the measuring circuitry comprises a filter configured to receive a pair of differential inputs indicative of the voltage across the resistive element and the diode and to generate an output signal representing an integral of the voltage represented by the differential inputs over time.
6. The apparatus according to claim 5 , comprising a switch configured to select one of a first configuration and a second configuration in which a polarity of the pair of differential inputs of the filter is reversed compared to the first configuration.
7. The apparatus according to claim 6 , comprising timing control circuitry configured to control the switch to select the first configuration when the first current I 1 or the third current I 3 is supplied to the resistive element and the diode, and to select the second configuration when the second current I 2 is supplied to the resistive element and the diode.
8. The apparatus according to claim 5 , comprising timing control circuitry configured to control the current control circuitry to supply the first current to the resistive element and the diode in a first time period of duration t, to supply the second current to the resistive element and the diode in a second time period of duration 2 t, and to supply the third current to the resistive element and the diode in a third time period of duration t.
9. The apparatus according to claim 5 , wherein the measuring circuitry comprises an analogue-to-digital converter configured to generate a digital value representing the resistance R of the resistive element in dependence on the output signal of the filter;
comprising timing control circuitry configured to control the current control circuitry to switch between the first current, the second current and the third current at a greater frequency than a sampling rate of the analogue-to-digital converter.
10. The apparatus according to claim 1 , wherein the current control circuitry comprises a first current source to generate the first current I 1 , a second current source to generate the second current I 2 , and a third current source to generate the third current I 3 .
11. The apparatus according to claim 1 , wherein the current control circuitry comprises four current sources each to generate a current with a magnitude corresponding to the third current I 3 , and switching circuitry to vary how many of the four current sources are coupled to the resistive element and the diode, to switch the current supplied to the resistive element and the diode between the first current I 1 , the second current I 2 and the third current I 3 .
12. The apparatus according to claim 1 , wherein the resistive element comprises a temperature sensor and the resistance R is indicative of temperature.
13. A method for measuring a resistance R of a resistive element arranged in series with a diode; the method comprising:
switching a current supplied to the resistive element and the diode between a first current I 1 , a second current I 2 and a third current I 3 , where I 1 >I 2 >I 3 ; and
determining the resistance R of the resistive element in dependence on a first voltage V 1 across the resistive element and the diode when the first current I 1 is supplied, a second voltage V 2 across the resistive element and the diode when the second current I 2 is supplied, and a third voltage V 3 across the resistive element and the diode when the third current I 3 is supplied;
wherein the resistance R is determined by combining the first voltage V 1 , the second voltage V 2 and the third voltage V 3 to at least partially eliminate an error in the determined resistance R of the resistive element caused by a voltage drop across the diode;
wherein the resistive element is one of an array of resistive elements arranged in rows and columns, wherein each resistive element is individually addressable via a corresponding row line and a corresponding column line and is arranged in series with a diode between the corresponding row line and the corresponding column line.