An integrated voltage reference supply circuit for an analog to digital converter, ADC, the voltage reference supply circuit comprising: an input node that, in use, receives an external voltage reference signal (Vref); a battery capacitor; a low-bandwidth amplifier receiving a voltage reference signal (Vref_amp) at a first one of its signal input and configured to supply, at an output node, the voltage on the external voltage reference signal (Vref) to the battery capacitor, wherein for a given converter sampling rate the amplifier, is of lower bandwidth than would need to be the case if the battery capacitor is not provided; and switching circuitry (Sample, S5) arranged to switch selectively an internal node (Vref_internal) between the input node, and the battery capacitor, so as to directly receive the external voltage reference signal (Vref) by connecting the internal node directly to the input node, the internal node (Vref_internal) being the reference input of a DAC forming part of the ADC; wherein prior to an operating phase of the ADC that requires a reference signal at the internal node (Vref_internal), the switching circuitry, (Sample, S5) is firstly operated, during a first phase of operation, so as to disconnect the internal node (Vref_internal) from the input node and connect it to the battery capacitor, whereby the battery capacitor supplies charge to the internal node (Vref) to bring it towards or maintain it at the voltage on the external voltage reference signal (Vref) and then secondly, the switching circuitry (Sample, S5) is operated during a second phase of operation, after the first phase, to connect the internal node (Vref_internal) directly to the input node to receive the external voltage reference signal (Vref) directly, and to disconnect the battery capacitor from the internal node (Vref_internal) wherein the amplifier recharges the battery capacitor towards the voltage on the external voltage reference signal (Vref). an input node that, in use, receives an external voltage reference signal (Vref); a battery capacitor; a low-bandwidth amplifier receiving a voltage reference signal (Vref_amp) at a first one of its signal input and configured to supply, at an output node, the voltage on the external voltage reference signal (Vref) to the battery capacitor, wherein for a given converter sampling rate the amplifier, is of lower bandwidth than would need to be the case if the battery capacitor is not provided; and switching circuitry (Sample, S5) arranged to switch selectively an internal node (Vref_internal) between the input node, and the battery capacitor, so as to directly receive the external voltage reference signal (Vref) by connecting the internal node directly to the input node, the internal node (Vref_internal) being the reference input of a DAC forming part of the ADC; wherein prior to an operating phase of the ADC that requires a reference signal at the internal node (Vref_internal), the switching circuitry, (Sample, S5) is firstly operated, during a first phase of operation, so as to disconnect the internal node (Vref_internal) from the input node and connect it to the battery capacitor, whereby the battery capacitor supplies charge to the internal node (Vref) to bring it towards or maintain it at the voltage on the external voltage reference signal (Vref) and then secondly, the switching circuitry (Sample, S5) is operated during a second phase of operation, after the first phase, to connect the internal node (Vref_internal) directly to the input node to receive the external voltage reference signal (Vref) directly, and to disconnect the battery capacitor from the internal node (Vref_internal) wherein the amplifier recharges the battery capacitor towards the voltage on the external voltage reference signal (Vref).