A medical system comprising: a probe configured to be inserted into a heart of a patient, at least one patch comprising: a substrate, configured to attach externally to the patient; an electrode (28, 52), which is coupled to the substrate and is configured to sense electrocardiogram signals from the heart of the patient, and to further sense electrical signals indicative of an impedance between the electrode (28, 52) and the probe in the heart; and circuitry, which is coupled to the substrate and comprises a shared amplifier that is configured to simultaneously amplify the ECG signals and the electrical signals sensed by the electrode (28, 52), wherein the circuitry further comprises first and second filters, which are configured to filter amplified ECG signals and amplified electrical signals, respectively, from an output of the amplifier, a console comprising an interface for receiving the filtered amplified ECG and the filtered amplified electrical signals sensed by the electrode of the patch, and comprising a processor configured to estimate the relative location and orientation of the probe relative to the electrode of the at least one patch, and further configured to process the filtered amplified ECG signals, wherein the patch is configured to output the filtered amplified ECG signals and filtered amplified electrical signals from the patch to the processor via at least a lead or cable. a probe configured to be inserted into a heart of a patient, at least one patch comprising: a substrate, configured to attach externally to the patient; an electrode (28, 52), which is coupled to the substrate and is configured to sense electrocardiogram signals from the heart of the patient, and to further sense electrical signals indicative of an impedance between the electrode (28, 52) and the probe in the heart; and circuitry, which is coupled to the substrate and comprises a shared amplifier that is configured to simultaneously amplify the ECG signals and the electrical signals sensed by the electrode (28, 52), wherein the circuitry further comprises first and second filters, which are configured to filter amplified ECG signals and amplified electrical signals, respectively, from an output of the amplifier, a substrate, configured to attach externally to the patient; an electrode (28, 52), which is coupled to the substrate and is configured to sense electrocardiogram signals from the heart of the patient, and to further sense electrical signals indicative of an impedance between the electrode (28, 52) and the probe in the heart; and circuitry, which is coupled to the substrate and comprises a shared amplifier that is configured to simultaneously amplify the ECG signals and the electrical signals sensed by the electrode (28, 52), wherein the circuitry further comprises first and second filters, which are configured to filter amplified ECG signals and amplified electrical signals, respectively, from an output of the amplifier, a console comprising an interface for receiving the filtered amplified ECG and the filtered amplified electrical signals sensed by the electrode of the patch, and comprising a processor configured to estimate the relative location and orientation of the probe relative to the electrode of the at least one patch, and further configured to process the filtered amplified ECG signals, wherein the patch is configured to output the filtered amplified ECG signals and filtered amplified electrical signals from the patch to the processor via at least a lead or cable.