Fingerprint sensor with sync signal input

A fingerprint sensor for sensing a fingerprint pattern of a finger and providing a fingerprint pattern signal indicative of the fingerprint pattern to external circuitry, said fingerprint sensor comprising:

• a voltage supply interface for receiving a supply voltage referenced to a time-varying sensor ground potential, said sensor ground potential varying between a relatively low first potential and a relatively high second potential in relation to a device ground potential being a reference potential for said external circuitry and for said finger;
• a sensor communication interface for receiving signals from said external circuitry and for providing said fingerprint pattern signal to said external circuitry;
• a synchronization input for receiving from said external circuitry a sync signal exhibiting a substantially constant sync signal potential, relative to said device ground potential, said sync signal potential being sufficiently close to said second potential to be interpreted by said fingerprint sensor to correspond to a first logical state when said sensor ground potential is at said first potential and to a second logical state, different from the first logical state, when said sensor ground potential is at said second potential;
• a plurality of sensing elements, each comprising:
• a protective dielectric top layer to be touched by said finger;
• an electrically conductive sensing structure arranged underneath said top layer;
• and
• a charge amplifier connected to said sensing structure for providing a sensing signal indicative of a change of a charge carried by said sensing structure resulting from a change in a potential difference between said finger and said sensing structure, said charge amplifier comprising:
• a negative input connected to said sensing structure;
• a positive input connected to a sensing element reference potential being substantially constant relative to said time-varying sensor ground potential;
• an output providing said sensing signal;
• a feedback capacitor connected between said negative input and said output;
• and
• at least one amplifier stage between said positive and negative inputs, and said output,
• wherein said charge amplifier is configured in such a way that a potential at said negative input substantially follows a potential at said positive input, such that said sensing element reference potential provides said change in potential difference between said finger and said sensing structure;
• and
• read-out circuitry connected to said synchronization input, and to the output of the charge amplifier of each of said sensing elements for sampling said sensing signal provided by each of said sensing elements at sampling times related to transitions, perceived by said fingerprint sensor, of said sync signal from said first logical state to said second logical state or from said second logical state to said first logical state, and forming said fingerprint pattern signal based on said sampled sensing signals.