Measuring electrode impedance in an impedance measurement circuit

Abstract

The disclosure provides a circuit for impedance measurement. The circuit includes an excitation source coupled between a first set of input switches. An impedance network is coupled between the first set of input switches and a first set of output switches. The impedance network includes a body impedance and a plurality of electrode impedances. A sense circuit is coupled to the first set of output switches. The sense circuit measures the body impedance and at least one electrode impedance of the plurality of electrode impedances.

What is claimed is: 
     
         1 . A circuit comprising:
 an excitation source coupled between a first set of input switches;   an impedance network coupled between the first set of input switches and a first set of output switches, the impedance network having a body impedance and a plurality of electrode impedances; and   a sense circuit coupled to the first set of output switches, the sense circuit configured to measure the body impedance and at least one electrode impedance of the plurality of electrode impedances.   
     
     
         2 . The circuit of  claim 1 , wherein the sense circuit is configured to generate an indication signal when at least one electrode impedance of the plurality of electrode impedances is above a defined threshold. 
     
     
         3 . The circuit of  claim 1 , wherein the sense circuit is configured to determine a an accurate value of the body impedance when each electrode impedance of the plurality of electrode impedances is below the defined threshold, and wherein the sense circuit is configured to determine the accurate value of the body impedance from the body impedance and the plurality of electrode impedances. 
     
     
         4 . The circuit of  claim 1  further comprising:
 a second set of input switches coupled in parallel to the first set of input switches, wherein the excitation source is coupled between the second set of input switches; and 
 a second set of output switches coupled between the impedance network and the sense circuit. 
 
     
     
         5 . The circuit of  claim 1 , wherein the plurality of electrode impedances comprises:
 a first input electrode impedance coupled to a first excitation terminal;   a second input electrode impedance coupled to a second excitation terminal;   
       wherein the body impedance is coupled between the first input electrode impedance and the second input electrode impedance;
 a first output electrode impedance coupled to a first sense terminal; and 
 a second output electrode impedance coupled to a second sense terminal. 
 
     
     
         6 . The circuit of  claim 1 , wherein the first set of input switches comprises:
 a first input switch coupled between the excitation source and the first excitation terminal; and   a second input switch coupled between the excitation source and the second excitation terminal.   
     
     
         7 . The circuit of  claim 1 , wherein the second set of input switches comprises:
 a third input switch coupled between the excitation source and the first sense terminal; and   a fourth input switch coupled between the excitation source and the second sense terminal.   
     
     
         8 . The circuit of  claim 1 , wherein the first set of output switches comprises:
 a first output switch and a second output switch coupled between the first sense terminal and the sense circuit; and   a third output switch and a fourth output switch coupled between the second sense terminal and the sense circuit.   
     
     
         9 . The circuit of  claim 1 , wherein the second set of output switches comprises:
 a fifth output switch and a sixth output switch coupled between the first excitation terminal and the sense circuit; and   a seventh output switch and an eighth output switch coupled between the second excitation terminal and the sense circuit.   
     
     
         10 . The circuit of  claim 1 , wherein the sense circuit is configured to measure the body impedance when the first input switch, the second input switch, the first output switch and the fourth output switch are closed. 
     
     
         11 . The circuit of  claim 1 , wherein the sense circuit is configured to measure:
 the first input electrode impedance when the first input switch, the second input switch, the second output switch and the fifth output switch are closed; and   the second input electrode impedance when the first input switch, the second input switch, the third output switch and the eighth output switch are closed.   
     
     
         12 . The circuit of  claim 1 , wherein the sense circuit is configured to measure:
 the first output electrode impedance when the third input switch, the fourth input switch, the first output switch and the sixth output switch are closed; and   the second output electrode impedance when the third input switch, the fourth input switch, the fourth output switch and the seventh output switch are closed.   
     
     
         13 . A method comprising:
 measuring a body impedance;   measuring at least one electrode impedance of a plurality of electrode impedances;   generating an indication signal when at least one electrode impedance of the plurality of electrode impedances is above a defined threshold; and   determining an accurate value of the body impedance when each electrode impedance of the plurality of electrode impedances is below the defined threshold, wherein the accurate value of the body impedance is determined from the body impedance and the plurality of electrode impedances.   
     
     
         14 . The method of  claim 13 , wherein measuring the body impedance further comprises closing a first input switch, a second input switch, a first output switch and a third output switch. 
     
     
         15 . The method of  claim 13 , wherein measuring at least one electrode impedance further comprises:
 closing the first input switch, the second input switch, a second output switch and a fifth output switch, to measure a first input electrode impedance;   closing the first input switch, the second input switch, a fourth output switch and a seventh output switch, to measure a second input electrode impedance;   closing a third input switch, a fourth input switch, the first output switch and a sixth output switch, to measure a first output electrode impedance; and   closing the third input switch, the fourth input switch, the third output switch and an eighth output switch, to measure a second output electrode impedance.   
     
     
         16 . The method of  claim 13 , wherein determining the accurate value of the body impedance further comprises using the body impedance, the first input electrode impedance, the second input electrode impedance, the first output electrode impedance and the second output electrode impedance to determine the accurate value of the body impedance, and each of the first input electrode impedance, the second input electrode impedance, the first output electrode impedance and the second output electrode impedance is below the defined threshold. 
     
     
         17 . The method of  claim 13  further comprising generating a current signal by an excitation source coupled between the first input switch and the second input switch, wherein the excitation source is coupled between the third input switch and the fourth input switch. 
     
     
         18 . The method of  claim 13 , wherein the body impedance coupled between the first input electrode impedance and the second input electrode impedance. 
     
     
         19 . A computing device comprising:
 a processing unit;   a memory module coupled to the processing unit; and   a impedance measurement circuit coupled to the processing unit and the memory module, the impedance measurement circuit comprising:
 an excitation source coupled between a first set of input switches; 
 an impedance network coupled between the first set of input switches and a first set of output switches, the impedance network having a body impedance and a plurality of electrode impedances; and 
 a sense circuit coupled to the first set of output switches, the sense circuit configured to measure the body impedance and at least one electrode impedance of the plurality of electrode impedances. 
   
     
     
         20 . The computing device of  claim 19  further comprises a plurality of electrodes, the plurality of electrodes includes a first excitation electrode, a second excitation electrode, a first sense electrode and a second sense electrode. 
     
     
         21 . The computing device of  claim 19 , wherein the plurality of electrode impedances comprises:
 a first input electrode impedance coupled to a first excitation terminal, the first input electrode impedance is corresponding to the first excitation electrode;   a second input electrode impedance coupled to a second excitation terminal, the second input electrode impedance is corresponding to the second excitation electrode, wherein the body impedance is coupled between the first input electrode impedance and the second input electrode impedance   a first output electrode impedance coupled to a first sense terminal, the first output electrode impedance is corresponding to the first sense electrode; and   a second output electrode impedance coupled to a second sense terminal, the second output electrode impedance is corresponding to the second sense electrode.   
     
     
         22 . The computing device of  claim 19 , wherein the sense circuit is configured to:
 generate an indication signal when at least one electrode impedance of the plurality of electrode impedances is above a defined threshold; and   determine an accurate value of the body impedance when each electrode impedance of the plurality of electrode impedances is below the defined threshold, and wherein the sense circuit is configured to determine the accurate value of the body impedance from the body impedance and the plurality of electrode impedances.