Current-output digital-to-analog converter

1 . A circuit comprising:

a digital-to-analog converter (DAC) configured to receive a binary input signal and to provide an output voltage that corresponds to a magnitude of the binary input signal, the DAC comprising a resistor string and a buffer stage comprising a first transistor configured to generate a string current that passes through the resistor string, the resistor string being configured to deliver an output voltage in accordance with the binary input signal;

a transconductance stage configured to receive the output voltage from the DAC and to generate a DAC current based on a magnitude of the output voltage; and

an auxiliary current generator comprising a second transistor having a gate node coupled to a gate node of the first transistor, the second transistor being matched to the first transistor, the auxiliary current generator configured to generate an auxiliary current;

wherein the DAC current and the auxiliary current are summed together to produce an output current.

2 . The circuit of claim 1 , wherein the DAC comprises a multiplexer having a plurality of inputs coupled along the resistor string and selection inputs configured to receive the binary input signal.

3 . The circuit of claim 2 , further comprising a mirror to generate the DAC current based on a replica of the current Idac and to supply the DAC current to a node for adding the auxiliary current to the DAC current.

4 . The circuit of claim 1 , wherein the transconductance stage comprises a resistor matched to a combined resistance of the resistor string.

5 . The circuit of claim 1 , wherein the auxiliary current generator comprises a current mirror having a first transistor with a first width/length (W/L) ratio and a second transistor with a second W/L ratio different from the first W/L ratio, wherein the current mirror is configured to scale the auxiliary current by a constant based on a ratio between the first and second W/L ratios.

6 . The circuit of claim 5 , wherein the auxiliary current represents a most significant bit (MSB) portion of the output current.

7 . The circuit of claim 1 , further comprising a gain stage configured to receive the output current.

8 . The circuit of claim 7 , wherein the gain stage includes a current mirror arrangement having a first transistor with a first width/length (W/L) ratio and a second transistor with a second W/L ratio different from the first W/L ratio, wherein the current mirror is configured to scale the output current by a constant based on a ratio between the first and second W/L ratios.

9 . The circuit of claim 1 , wherein the DAC is a coarse-fine DAC that includes a coarse stage and a fine stage.

10 . The circuit of claim 1 , wherein the auxiliary current generator includes a transconductor configured to generate the auxiliary current.

11 . A photon counting system comprising:

a photon detector configured to generate a current pulse in response to receiving one or more photons; and

a current-output digital-to-analog converter (DAC) configured to compensate for a leakage current associated with the photon detector, the current-output digital-to-analog converter (DAC) comprising a voltage-output DAC configured to receive a binary input signal and to provide an output voltage that corresponds to a magnitude of the binary input signal, the DAC comprising a resistor string and a buffer stage comprising a first transistor configured to generate a string current that passes through the resistor string, the resistor string being configured to deliver an output voltage in accordance with the binary input signal;

a transconductance stage configured to receive the output voltage from the DAC and to generate a DAC current based on a magnitude of the output voltage; and

an auxiliary current generator comprising a second transistor having a gate node coupled to a gate node of the first transistor, the second transistor being matched to the first transistor, the auxiliary current generator configured to generate an auxiliary current based on a replica of the string current;

wherein the DAC current and the auxiliary current are summed together to produce an output current.

12 . The photon counting system of claim 11 , further comprising:

front-end circuitry configured to convert the current pulse into a voltage pulse;

a discriminator circuit comprising a plurality of comparators and configured to receive the voltage pulse and determine a relative amplitude of the voltage pulse; and

one or more counters configured to count a number of received voltage pulses having different heights.

13 . A digital-to-analog converter (DAC) circuit, comprising:

a voltage-output DAC configured to receive a binary input signal and generate a corresponding output voltage, the voltage-output DAC comprising a multiplexer, a resistor string, and a buffer stage, wherein the buffer stage includes a first transistor configured to provide a string current across the resistor string;

a transconductance stage configured to receive the output voltage from the voltage-output DAC and to generate a DAC current based on a magnitude of the output voltage; and

an auxiliary current generator comprising a second transistor configured to provide an auxiliary current based on a replica of the string current;

wherein the DAC current and the auxiliary current are summed together to produce an output current.

14 . The DAC circuit of claim 13 , wherein the multiplexer comprises a plurality of inputs coupled along the resistor string and selection inputs configured to receive the binary input signal.

15 . The DAC circuit of claim 13 , wherein a gate node of the first transistor is coupled to a gate node of the second transistor.

16 . The DAC circuit of claim 13 , further comprising a gain stage configured to receive the output current.