FIELD OF THE DISCLOSURE

The present disclosure relates to current-to-voltage signal converters. In particular, it relates to a circuit in which the operational voltage may be adjusted, such that low-voltage components may be used.

BACKGROUND

In many applications it is necessary to convert current signals into voltage signals. A way to do this is with a trans-impedance amplifier.

Within the technical field of optical sensing it a trans-impedance amplifier can convert an output current from an optical photodiode to a voltage signal ready to be converted by an analog-to-digital converter (ADC). For some applications the output voltage range from a trans-impedance amplifier is further conditioned to make it suitable for being converted by an ADC. For example, by making the dynamic range of the signal match the dynamic range of the ADC.

Optical photodiodes are often biased with a voltage reference and the output current signal of the optical photodiode will be dependent on the voltage reference used. Therefore, it is possible to directly adjust the voltage reference in order to bias the output current signal. However, this can result in excess components and can often impact upon the accuracy of the representation of the original optical signal if the voltage reference is not accounted for.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a current-to-voltage signal converter which may operate at a reduced voltage. The current-to-voltage converter includes a trans-impedance amplifier which converts a current input into a voltage output. The voltage output may operate above a first predetermined voltage or below a second predetermined voltage (where the first predetermined voltage is greater than the second predetermined voltage), and must therefore be adjusted in order to make it suitable for any downstream signal processing circuitry, such as an ADC. As such, a subtractor circuit is coupled to the output of the trans-impedance amplifier. At the input of the subtractor circuit, a voltage adjustment circuit is employed, to adjust the voltage input to the subtractor circuit. As such, the input to the subtractor is adjusted between a first predetermined voltage threshold and a second predetermined voltage threshold, and the subtractor circuit may therefore be a low-voltage component.

According to a first aspect there is provided a current-to-voltage signal converter, comprising: a first stage, configured to convert an input current signal to a first voltage signal; an analog subtractor circuit comprising an amplifier, wherein the analog subtractor circuit is electrically coupled to the first stage to convert the first voltage signal into a second voltage signal; and, a voltage adjustment arrangement, coupled to at least one input of the amplifier, and configured such that the voltage at the at least one input of the amplifier is between a first predetermined voltage threshold and a second predetermined voltage threshold.

The voltage adjustment arrangement can allow for an improvement is system cost (reduced component size, power) and an improved system performance. This can allow the amplifier to be a low voltage amplifier and/or a unipolar amplifier which can lead to further advantages.

A low voltage amplifier operates with a lower supply voltage than an amplifier. This is a relative concept which, for an integrated circuit (IC), depends on the foundry process. In a foundry process, it is possible to select different voltage ranges and some are lower, some are higher. An IC can comprise many different discrete voltage levels e.g. 20V, 10V, 5V, 3.3V, 1.8V, 1.3V, etc. To differentiate the two types of amplifiers, an amplifier in an IC can operate with a supply voltage which can be the highest voltage level supported by that IC. A low voltage amplifier in an IC can operate with a supply voltage lower than the highest voltage level supported by that IC, For example, on one IC the highest voltage level supported may be 10V, therefore a low voltage amplifier may operate with a supply voltage of 5V. In another example, on one IC the highest voltage level supported may be 5V, therefore a low voltage amplifier may operate with a supply voltage of 1.8V.

This can lead to accuracy and power advantages for the low voltage amplifier. Thus, a low voltage amplifier can operate at (or have a supply voltage) an IC supported voltage level less than the maximum voltage level supported. Alternatively, a low voltage amplifier can operate at (or have a supply voltage) the lowest voltage level supported by an IC.

Similarly, an amplifier may operate with an asymmetrical supply voltage, such that the positive supply voltage can be the highest voltage level supported by that IC and a negative supply voltage which can be 0V. The reduced dynamic range of this amplifier supplied by an asymmetrical supply voltage may also lead to accuracy and power advantages.

It is also possible to arrange the voltage adjustment arrangement to bias the voltage level so that the inputs to the amplifier is a positive voltage value. This allows the amplifier to be supplied by a unipolar supply voltage i.e. the negative voltage rail is set to electrical ground. This enables the system to operate with greater power efficiency.

Thus the amplifier may be a low voltage amplifier and/or supplied by a unipolar supply voltage. This can improve system performance (accuracy, power efficiency) and reduce cost of the system (reduced surface area and size).

The voltage adjustment arrangement can comprise a first reference source, coupled to the at least one input of the amplifier.

The at least one input of the amplifier can comprise a first input and a second input. The first reference source can be coupled to both the first input and the second input, and can be a common mode reference.

The common mode reference can be a voltage or current source.

The first predetermined voltage threshold is greater than the second predetermined voltage threshold, wherein the first voltage signal is at least partially above the first predetermined voltage threshold or at least partially below the second predetermined voltage threshold.

The amplifier can be a low voltage amplifier and can operate in its linear region. The amplifier can have a predetermined input voltage range, and the first predetermined voltage threshold can be less than or equal to the upper end of the predetermined input voltage range, and the second predetermined voltage threshold is greater than or equal to the lower end of the predetermined input voltage range. The first and second predetermined voltage thresholds may be substantially equal to the positive and negative supply voltage of amplifier divided by the gain of the amplifier, respectively.

The first stage can be configured to operate within a first supply voltage range. The amplifier can be an operational amplifier and can be configured to operate within a second supply voltage range. The first supply voltage range can be greater than the second supply voltage range. The amplifier can be configured to be further powered by a unipolar supply voltage. The positive and negative supply voltage of the amplifier can be different from the positive and negative supply voltage of the first stage. The dynamic range of the amplifier can be less than the dynamic range of the first stage. The voltage range between the positive and negative supply voltage of the amplifier is less than the voltage range between the positive and negative supply voltage of the first stage.

The input current signal can be generated by a photodiode biased by a first reference voltage. The first stage can be a trans-impedance amplifier comprising a voltage input and a current input. The first reference voltage can be electrically coupled to the voltage input and the input current signal can be electrically coupled to the current input. The second voltage signal can be suitable for supplying an Analog to Digital Converter.

The analog subtractor circuit can further comprise a first, second, third and fourth resistors. The first resistor can be electrically coupled in series between the first input and the first voltage signal. The second resistor can be electrically coupled to the first input and forms a negative feedback loop based on the output of the operational amplifier. The output of the operational amplifier can be the second voltage signal. The third resistor and the fourth resistor can form a potential divider such that the third resistor can be electrically coupled in series between the second input and the first reference voltage, and the fourth resistor can be electrically coupled to a ground state. The first and third resistors can have substantially the same value resistance and the second and fourth resistors can have substantially the same value resistance.

The common mode reference can comprise a/the fixed voltage source electrically coupled to a first terminal of a first common mode resistor and a first terminal of a second common mode resistor. The second terminals of the first and second common mode resistors can be electrically coupled to the first and second inputs respectively. The first and second common mode resistors can have substantially the same value resistance.

The first reference voltage can have a predetermined value, and the common mode reference is a fixed voltage source can have a predetermined voltage based on the predetermined value.

The voltage adjustment arrangement can comprise a feedback loop configured to adjust the voltage of both the first input and second input into the amplifier of the analog subtractor circuit. The voltage adjustment arrangement can comprise a feedback amplifier configured to receive the second input to the amplifier of the analog subtractor circuit.

The feedback amplifier can be an NMOS transistor. The voltage of both the first input and the second input into the amplifier of the analog subtractor circuit can correspond to at least the threshold voltage of the NMOS transistor.

The feedback amplifier can be electrically coupled to a first terminal of a first common mode resistor and to a first terminal of a second common mode resistor. The second terminals of first and second common mode resistors can be electrically coupled to the first and second input respectively. The first and second common mode resistors can have substantially the same value resistance.

The feedback amplifier can be configured to operate a current mirror arrangement comprising two mirrored transistors and a control transistor. The feedback amplifier can be arranged to control the current flow in the control transistor and the two mirrored transistors can be electrically coupled to the first and second input respectively. The two mirrored transistors can be size matched.

The first stage can include a first reference voltage. The first voltage signal can be dependent on the first voltage reference. The second voltage signal can be independent of the first voltage reference.

According to a second aspect there is provided a transimpedance amplifier, comprising: a current-to-voltage converter, configured to receive a first current signal, and to generate a positive voltage output; a voltage adjustment arrangement, coupled to the current-to-voltage converter, and configured to enable at least part of the current-to-voltage converter to between a first predetermined voltage threshold and a second predetermined voltage threshold. The voltage adjustment arrangement can comprise a constant voltage/current source coupled to a first terminal of a resistor. A second terminal of the resistor can be coupled to the current-to-voltage converter.

The voltage adjustment arrangement can comprise a feedback amplifier arranged to actively control the reduction in voltage based on the positive voltage output.

According to a third aspect there is provided a method of converting a current signal to a voltage signal, the method comprising: converting an input current signal to a first voltage signal using a first stage of a signal converter; converting the first voltage signal into a second voltage signal using an analog subtractor circuit comprising an amplifier; and, applying a voltage adjustment arrangement to at least one input of the amplifier to reduce the voltage at the at least one input to the amplifier between a first predetermined voltage threshold and a second predetermined voltage threshold.

The at least one input of the amplifier can comprise a first input and a second input. The voltage adjustment arrangement can comprise a common mode reference coupled to both the first input and the second input. The common mode reference can be a DC source.

The method can further include performing the step of predetermining a DC offset voltage of the first voltage signal. The method can further performing the step of predetermining a voltage or current value of the DC source based on the predetermined DC offset voltage.

The method can further include performing the step of adjusting a voltage or current value at both the first input and the second input based on a DC offset voltage of the first voltage signal.

The method can further include performing the step of receiving the input current signal from a photodiode. The method can further performing the step of applying the second voltage signal to an Analog to Digital Converter.

