Closed loop dynamic capacitance measurement
This disclosure provides systems, methods and apparatus for measuring capacitance of a display unit, such as an interferometric modulator (IMOD). In one example, a circuit may include an operational amplifier (op-amp), a voltage controlled current source, and feedback from an output of the op-amp as an input to the voltage controlled current source. An output of the voltage controlled current source may be provided to a display unit as well as an input of the op-amp. A second input of the op-amp may be provided a ramping reference voltage.
1. A circuit comprising:
an amplifier having a first input, a second input, and an output, the first input of the amplifier coupled with a voltage reference source;
a current source having an input and an output, the input of the current source coupled with the output of the amplifier, the output of the current source coupled with the second input of the amplifier, the current source being voltage controlled; and
one or more display units having a first terminal coupled with the second input of the amplifier and with the output of the current source, wherein the current source is configured to provide a current based on a voltage associated with the output of the amplifier, the current increasing until a voltage associated with the first terminal of the one or more display units equals a voltage associated with the voltage reference source, wherein the first terminal of the one or more display units is associated with a movable element, and the voltage reference source is configured to ramp through a voltage range associated with a travel range of the movable element.
2. The circuit of claim 1 , further comprising:
a feedback capacitor having a first terminal and a second terminal, the first terminal of the feedback capacitor coupled with the output of the current source, the second terminal of the feedback capacitor coupled with the output of the amplifier.
3. The circuit of claim 2 , wherein a capacitance of the feedback capacitor is less than or equal to a capacitance of the one or more display units.
4. The circuit of claim 1 further comprising:
an analog-to-digital converter having an input coupled with the output of the amplifier.
5. The circuit of claim 1 , wherein the one or more display units have a second terminal set to a first fixed voltage.
6. The circuit of claim 5 , wherein the one or more display units have a third terminal set to the first fixed voltage.
7. The circuit of claim 5 , wherein the one or more display units have a third terminal set to a second fixed voltage.
8. The circuit of claim 1 , wherein the travel range of the movable element is between a second terminal and a third terminal of the display unit.
9. The circuit of claim 1 , wherein the current source has a linear gain.
10. The circuit of claim 1 , wherein a movable element associated with the first terminal of the one or more display units is configured to move to positions between a second terminal and a third terminal of the one or more display units until the voltage associated with the first terminal of the display units equals the voltage associated with the voltage reference source.
11. The circuit of claim 1 , further comprising:
a display including a plurality of display units;
a processor that is configured to communicate with the display, the processor being configured to process image data; and
a memory device that is configured to communicate with the processor.
12. The circuit of claim 11 , further comprising:
a driver circuit configured to send at least one signal to the display; and
a controller configured to send at least a portion of the image data to the driver circuit.
13. The circuit of claim 11 , further comprising:
an image source module configured to send the image data to the processor, wherein the image source module comprises at least one of a receiver, transceiver, and transmitter.
14. The circuit of claim 11 , further comprising:
an input device configured to receive input data and to communicate the input data to the processor.
15. A system comprising:
a measurement circuit including:
an amplifier having a first input, a second input, and an output, the first input of the amplifier coupled with a voltage reference source, and
a current source having an input and an output, the input of the current source coupled with the output of the amplifier, the output of the current source coupled with the second input of the amplifier, the current source being voltage controlled;
one or more display units having a first terminal coupled with the second input of the amplifier and with the output of the current source, wherein the current source is configured to provide a current based on a voltage associated with the output of the amplifier, the current increasing until a voltage associated with the first terminal of the one or more display units equals a voltage associated with the voltage reference source, wherein the first terminal of the one or more display units is associated with a movable element, and the voltage reference source is configured to ramp through a voltage range associated with a travel range of the movable element;
an analog-to-digital converter (ADC) having an input coupled with the output of the amplifier, and an output to provide measurement data; and
a memory configured to store the measurement data.
16. The system of claim 15 , further comprising:
a microcontroller configured to analyze the measurement data.
17. A method for determining capacitance of a group of one or more display units, the method comprising:
ramping a reference voltage associated with a first input of an amplifier, the amplifier having a second input, and an output;
generating a current from a current source having an input and an output, the input of the current source coupled with the output of the amplifier, the output of the current source coupled with the second input of the amplifier, the current source being voltage controlled; and
providing a voltage associated with a first terminal of the group of one or more display units, the first terminal coupled with the second input of the amplifier and with the output of the current source, wherein the current source provides a current based on a voltage associated with the output of the amplifier, the current increasing until the voltage associated with the first terminal of the one or more display units equals a voltage associated with the voltage reference source, wherein the first terminal of the group of one or more display units is associated with a movable element, and the reference voltage is ramped through a voltage range associated with a travel range of the movable element.
18. The method of claim 17 , the method further comprising:
providing a feedback capacitor, the feedback capacitor having a first terminal coupled with the output of the current source and a second terminal coupled with the output of the amplifier.
19. The method of claim 18 , the method further comprising:
changing a capacitance of the feedback capacitor based on a number of display units in the group of the one or more display units.