Method of driving an element of an active matrix EWOD device, a circuit, and an active matrix EWOD device
A method of driving an element of an active matrix electro-wetting on dielectric (AM-EWOD) device comprise applying a first alternating voltage to a reference electrode of the AM-EWOD device; and either (i) applying to the element electrode a second alternating voltage that has the same frequency as the first alternating voltage and that is out of phase with the first alternating voltage or (ii) holding the element electrode in a high impedance state. The effect of applying the second alternating voltage to the element electrode is to put the element in an actuated state in which the element is configured to actuate any liquid droplet present in the element, while the effect of holding the element electrode in the high impedance state is to put the element in a non-actuated state.
1. A circuit for selectively providing an alternating voltage to an AM-EWOD element electrode, the circuit having a memory element, an input node for connection to a source of an alternating voltage, an output node for connection to the AM-EWOD element electrode, and a first switch for addressing the output node by, in dependence on a data value stored in the memory element, any one of electrically connecting the input node to the output node and electrically isolating the input node from the output node;
wherein the circuit is configured such that when the output node is electrically isolated from the input node, the output node is at a floating potential with the element electrode placed in a high impedance state.
2. A circuit as claimed in claim 1 , wherein the first switch is connected between the input node and the output node, a control terminal of the first switch being connected to an output of the memory element, wherein the first switch is a first transistor, and wherein the first transistor is of a low leakage design comprising a Lightly Doped Drain (LDD) transistor.
3. A circuit as claimed in claim 1 wherein the memory element comprises:
a second switch connected between a data input and the control terminal of the first switch, a control terminal of the second switch being connected to a first control input; and
a first capacitor connected between the control terminal of the first switch and a bias voltage.
4. A circuit as claimed in claim 3 wherein the second switch is a second transistor.
5. A circuit as claimed in claim 1
wherein the memory element comprises:
a second switch connected between a data input and the control terminal of the first switch, a control terminal of the second switch being connected to a first control input; and
a first capacitor connected between the control terminal of the first switch and a bias voltage;
wherein the second switch is a second transistor; and
wherein the first transistor and the second transistor are transistors of the same channel type as one another.
6. A circuit as claimed in claim 1 and comprising a third switch connected between a source of the second alternating voltage and the output node, a control terminal of the third switch being connected to a second control input.
7. A circuit as claimed in claim 1 and comprising a second capacitor connected between the output node and a source of a bias voltage.
8. A circuit as claimed in claim 1 and comprising a third capacitor connected between the control terminal of the first switch and the input node, wherein the third capacitor has a voltage-dependent capacitance.
9. A circuit as claimed in claim 1 wherein the memory element comprises a static read-only memory (SRAM).
10. A circuit as claimed in claim 1 and further comprising a sensor circuit connected between the output node and a sensing output node.
11. An active matrix EWOD device having a plurality of AM-EWOD elements, each element having an element electrode and a reference electrode, the device comprising a reference electrode drive circuit for applying a first alternating voltage to the reference electrode and array element circuits for addressing the element electrode of a respective AM-EWOD element by any one of (i) applying a second alternating voltage to the element electrode and (ii) putting the element electrode of the respective AM-EWOD element in a high impedance state, wherein the second alternating voltage has the same frequency as the first alternating voltage and the second alternating voltage is out of phase with the first alternating voltage,
wherein at least one of the array element circuits is an array element circuit as defined in claim 1 .
12. A circuit as claimed in claim 1 , further comprising a reference electrode spaced oppositely from the element electrode, wherein when the output node is electrically isolated from the input node, a voltage applied to the reference electrode is capacitively coupled to the element electrode when a droplet is present at the element electrode.
13. A circuit as claimed in claim 1 , wherein the input node further is connectable to a source of a second alternating voltage, and prior to the first switch switching from electrically connecting the input node to the output node to electrically isolating the input node from the output node, the second alternating voltage is applied to the output node to pre-charge the element electrode to the second alternating voltage.