Input control circuit for analog device
An input control circuit that can be used to drive analog switches of analog modules such as an analog-to-digital converter (ADC) enables a sampling switch to receive a higher input voltage than the voltage rating of the devices comprising the sampling switch without risk of damage and without the need for a resistor divider network. The input control circuit and switch both receive an input voltage to be processed and the input control circuit generates a control signal for the switch that is derived from a pre-charged capacitor. The control circuit permits the design and manufacture of high voltage analog modules using low voltage devices, which can save on mask costs without any performance trade-offs.
1. A control circuit for an analog device that includes a sampling switch arrangement that receives an analog voltage to be sampled and has a control input, the control circuit comprising:
an input terminal for receiving the analog voltage to be sampled;
an output terminal for connection to the control input of the sampling switch arrangement;
a first switch arrangement;
a charging capacitor operably coupled between the input terminal and the first switch arrangement; and
a charging circuit operably coupled to the charging capacitor for charging the charging capacitor to a preset voltage,
wherein the first switch arrangement connects the output terminal to ground in an ‘off’ mode of operation and transfers a voltage from a terminal of the charging capacitor to the output terminal in an ‘on’ mode of operation.
2. The control circuit of claim 1 wherein the first switch arrangement comprises:
a P MOSFET (P channel metal oxide semiconductor field effect transistor) connected in series with an N MOSFET (N channel MOSFET), for connecting the output terminal to ground by way of the N MOSFET in the ‘off’ mode and transferring a voltage from a terminal of the charging capacitor to the output terminal by way of the P MOSFET in the ‘on’ mode.
3. The control circuit of claim 2 , wherein the charging circuit sets a first terminal of the charging capacitor to a voltage equal to the received analog voltage plus a charging voltage, and sets a second terminal of the charging capacitor to the received analog voltage, and
wherein a source terminal of the P MOSFET is connected to the first terminal of the charging capacitor, and
wherein the control circuit includes a second switch arrangement that connects a gate terminal of the P MOSFET to a supply voltage in the ‘off’ mode and to the second terminal of the charging capacitor in the ‘on’ mode.
4. An analog device comprising a control circuit and a sampling switch, the control circuit comprising:
an input terminal for receiving an analog voltage to be sampled;
an output terminal;
a first switch;
a charging capacitor operably coupled between the input terminal and the first switch; and
a charging circuit operably coupled to the charging capacitor for charging the charging capacitor to a preset voltage,
wherein the first switch connects the output terminal to ground in an ‘off’ mode of operation and transfers a voltage from a terminal of the charging capacitor to the output terminal in an ‘on’ mode of operation, and
wherein the sampling switch comprises:
a first N MOSFET (metal oxide semiconductor field effect transistor) connected in series between second and third N MOSFETs,
wherein a drain terminal of the second N MOSFET receives the analog voltage to be sampled, a drain terminal of the third N MOSFET comprises an output of the sampling switch, gate terminals of the second and third N MOSFETs are connected to the charging capacitor, and a gate terminal of the first N MOSFET is connected to the output terminal of the control circuit.