An integrated voltage reference supply circuit for an analog to digital converter, the voltage reference supply circuit comprising: an input node that in use receives an external voltage reference signal Vref; a comparator (Comp1) receiving the voltage reference signal Vref at a first one of its signal inputs and an internal voltage reference signal Vref_internal present at a reference input of a DAC forming part of the analog to digital converter at a second one of its signal inputs, and further arranged to output a difference control signal in dependence on a difference between Vref and the internal voltage reference signal Vref_internal, the circuit being configured to provide a capacitance at the reference input of the DAC; a controllable switch receiving the difference control signal and switching charge to Vref_internal from a voltage supply (Vdd) in dependence thereon; and switching circuitry arranged to switch selectively the Vref_internal node to the input node to directly receive the voltage reference signal Vref; wherein prior to an operating phase of the ADC that requires a reference signal at the reference input the switching circuitry is firstly operated during a first phase of operation so as to disconnect the Vref_internal node from the reference signal Vref, and the comparator measures any difference between the voltages on Vref_internal and Vref and controls the controllable switch to supply charge to the Vref_internal node to bring it towards Vref, and then secondly the switch circuitry is operated during a second phase of operation after the first phase to connect the Vref_internal node to the input node to receive the external voltage reference signal Vref so that the Vref_internal node is brought finally to Vref for use as the reference signal required by the ADC,wherein the comparator has an operating threshold, for the detection of differences between Vref and Vref_internal, wherein the comparator continues to output an ON signal to switch the controllable switch for a finite time after the difference between Vref and Vref-internal is within the operating threshold, the voltage reference supply circuit further comprising threshold control circuitry configured to dynamically adjust the comparator operating threshold to a value that is a finite amount less than the external voltage reference signal Vref in dependence on the finite time to ensure that charge drawn from Vref resulting from an error between Vref_internal, as supplied by the controllable switch, and Vref is compensated. an input node that in use receives an external voltage reference signal Vref; a comparator (Comp1) receiving the voltage reference signal Vref at a first one of its signal inputs and an internal voltage reference signal Vref_internal present at a reference input of a DAC forming part of the analog to digital converter at a second one of its signal inputs, and further arranged to output a difference control signal in dependence on a difference between Vref and the internal voltage reference signal Vref_internal, the circuit being configured to provide a capacitance at the reference input of the DAC; a controllable switch receiving the difference control signal and switching charge to Vref_internal from a voltage supply (Vdd) in dependence thereon; and switching circuitry arranged to switch selectively the Vref_internal node to the input node to directly receive the voltage reference signal Vref; wherein prior to an operating phase of the ADC that requires a reference signal at the reference input the switching circuitry is firstly operated during a first phase of operation so as to disconnect the Vref_internal node from the reference signal Vref, and the comparator measures any difference between the voltages on Vref_internal and Vref and controls the controllable switch to supply charge to the Vref_internal node to bring it towards Vref, and then secondly the switch circuitry is operated during a second phase of operation after the first phase to connect the Vref_internal node to the input node to receive the external voltage reference signal Vref so that the Vref_internal node is brought finally to Vref for use as the reference signal required by the ADC,wherein the comparator has an operating threshold, for the detection of differences between Vref and Vref_internal, wherein the comparator continues to output an ON signal to switch the controllable switch for a finite time after the difference between Vref and Vref-internal is within the operating threshold, the voltage reference supply circuit further comprising threshold control circuitry configured to dynamically adjust the comparator operating threshold to a value that is a finite amount less than the external voltage reference signal Vref in dependence on the finite time to ensure that charge drawn from Vref resulting from an error between Vref_internal, as supplied by the controllable switch, and Vref is compensated.