Differential IDAC for PPG applications
According to an embodiment, a differential current digital-to-analog converter (IDAC) proposed. The differential IDAC includes first and second reference resistors respectively coupled to a voltage supply rail and reference ground. A reference current source is positioned between the two reference resistors. The converter also features first and second variable resistors, both coupled to the voltage supply rail, with resistance values based on respective digital codes. Additionally, a p-channel transistor sources a first output current from the first variable resistor to a first output terminal, while an n-channel transistor sources a second output current from the second variable resistor to a second output terminal. The system includes amplifiers to ensure voltage equality across both pairs of reference and variable resistors, thereby stabilizing the output currents.
1 . A differential current digital to analog converter (IDAC), the differential IDAC comprising:
a first reference resistor coupled to a voltage supply rail;
a second reference resistor coupled to reference ground;
a reference current source arranged between the first reference resistor and the second reference resistor;
a first variable resistor coupled to the voltage supply rail, the first variable resistor having a first resistance value based on a first digital code;
a second variable resistor coupled to the voltage supply rail, the second variable resistor having a second resistance value based on a second digital code;
a p-channel transistor configured to source a first output current from the first variable resistor to a first output terminal;
an n-channel transistor configured to source a second output current from the second variable resistor to a second output terminal;
a first amplifier configured to cause a first voltage across the first variable resistor to equal a first voltage across the first reference resistor; and
a second amplifier configured to cause a second voltage across the second variable resistor to equal a second voltage across the second reference resistor.
2 . The IDAC of claim 1 , wherein the reference current source comprises an operational amplifier and a second n-channel transistor.
3 . The IDAC of claim 2 , wherein the operational amplifier is configured to receive a low-noise reference voltage at a non-inverting input of the operational amplifier.
4 . The IDAC of claim 3 , wherein an inverting input of the operational amplifier is coupled to the non-inverting input of the second amplifier.
5 . The IDAC of claim 2 , wherein a drain terminal of the second n-channel transistor is coupled to the first reference resistor, a source terminal of the second n-channel transistor is coupled to the second reference resistor, and a control terminal of the second n-channel transistor is coupled to an output of the operational amplifier.
6 . The IDAC of claim 1 , wherein currents flowing at a drain terminal of the p-channel transistor and a drain terminal of the n-channel transistor provide compensation current to reduce an ambient light component of an electrical signal corresponding to a photoplethysmography (PPG) signal measurement.
7 . The IDAC of claim 6 , wherein the electrical signal corresponds to a light signal passed through or reflected from body tissue that is collected for the photoplethysmography (PPG) signal measurement.
8 . A system, comprising:
a detector configured to capture light and generate an electrical signal:
a photoplethysmography (PPG) circuit coupled to the detector, the PPG circuit configured to receive the electrical signal and generate compensation current for ambient light components of the electrical signal, the PPG circuit comprising:
a first reference resistor coupled to a voltage supply rail;
a second reference resistor coupled to reference ground;
a reference current source arranged between the first reference resistor and the second reference resistor;
a first variable resistor coupled to the voltage supply rail, the first variable resistor having a first resistance value based on a first digital code;
a second variable resistor coupled to the voltage supply rail, the second variable resistor having a second resistance value based on a second digital code;
a p-channel transistor configured to source a first output current from the first variable resistor to a first output terminal;
an n-channel transistor configured to source a second output current from the second variable resistor to a second output terminal;
a first amplifier configured to cause a first voltage across the first variable resistor to equal a first voltage across the first reference resistor; and
a second amplifier configured to cause a second voltage across the second variable resistor to equal a second voltage across the second reference resistor.
9 . The system of claim 8 , wherein the light captured by the detector is passed through body tissue.
10 . The system of claim 8 , wherein the light captured by the detector is reflected from body tissue.
11 . The system of claim 8 , wherein the reference current source comprises an operational amplifier and a second n-channel transistor, wherein the operational amplifier is configured to receive a low-noise reference voltage at a non-inverting input of the operational amplifier, and wherein an inverting input of the operational amplifier is coupled to the non-inverting input of the second amplifier.
12 . The system of claim 11 , wherein a drain terminal of the second n-channel transistor is coupled to the first reference resistor, a source terminal of the second n-channel transistor is coupled to the second reference resistor, and a control terminal of the second n-channel transistor is coupled to an output of the operational amplifier.
13 . The system of claim 8 , wherein currents flowing at a drain terminal of the p-channel transistor and a drain terminal of the n-channel transistor provide the compensation current to reduce an ambient light component of the electrical signal corresponding to a photoplethysmography (PPG) signal measurement.
14 . The system of claim 13 , wherein the electrical signal corresponds to a light signal passed through or reflected from body tissue that is collected for the PPG signal measurement.
15 . A method to generate a differential current to reduce an ambient light component in an electrical signal corresponding to a photoplethysmography (PPG) signal measurement, the method comprising:
sourcing, by a p-channel transistor, a first output current from a first variable resistor to a first output terminal, the first variable resistor coupled to a voltage supply rail, the first variable resistor having a first resistance value based on a first digital code;
sourcing, by an n-channel transistor, a second output current from a second variable resistor to a second output terminal, the second variable resistor coupled to the voltage supply rail, the second variable resistor having a second resistance value based on a second digital code;
setting, by a first amplifier, a first voltage across the first variable resistor to equal a first voltage across a first reference resistor, the first reference resistor coupled to the voltage supply rail; and
setting, by a second amplifier, a second voltage across the second variable resistor equal to a second voltage across a second reference resistor, the second reference resistor coupled to reference ground, a reference current source arranged between the first reference resistor and the second reference resistor.
16 . The method of claim 15 , wherein the reference current source comprises an operational amplifier and a second n-channel transistor.
17 . The method of claim 16 , wherein the operational amplifier is configured to receive a low-noise reference voltage at a non-inverting input of the operational amplifier, and wherein an inverting input of the operational amplifier is coupled to the non-inverting input of the second amplifier.
18 . The method of claim 17 , wherein a drain terminal of the second n-channel transistor is coupled to the first reference resistor, a source terminal of the second n-channel transistor is coupled to the second reference resistor, and a control terminal of the second n-channel transistor is coupled to an output of the operational amplifier.
19 . The method of claim 15 , wherein currents flowing at a drain terminal of the p-channel transistor and a drain terminal of the n-channel transistor provide a compensation current to reduce the ambient light component of the electrical signal corresponding to the PPG signal measurement.
20 . The method of claim 15 , wherein the electrical signal corresponds to a light signal passed through or reflected from body tissue that is collected for the PPG signal measurement.