Low power current to frequency converter circuit for use in implantable sensors

An implantable sensor comprising:

• a sensor that generates an analog current as a function of a substance or parameter sensed by the sensor;
• a current-to-frequency (I-to-F) converter circuit that converts the analog current generated by the sensor to a frequency signal, F.sub.OUT, having a frequency that varies as a function of the analog current, said I-to-F converter circuit comprising:
• an amplifier having a positive input terminal, a negative input terminal, and an output terminal, the amplifier including means for differentially amplifying an electrical signal applied between its two input terminals to provide an amplified output signal that appears on its output terminal;
• a first capacitor connected to one of the input terminals of the amplifier;
• a voltage controlled oscillator (VCO) circuit having a voltage-control input terminal connected to the output terminal of the amplifier, and a VCO output terminal, the VCO including means for generating the frequency signal F.sub.OUT as an output signal of the VCO, the signal F.sub.OUT having a frequency that varies as a function of the magnitude of a voltage applied to the voltage-control input;
• a charge-pump circuit coupled to the first capacitor that pumps a discrete charge off of the first capacitor under control of the frequency of the signal F.sub.OUT ;
• wherein the analog current from the sensor is applied to the first capacitor and tends to cause a charge to accumulate on the first capacitor as a function of the magnitude of the analog current, which charge tends to increase the output voltage of the amplifier so as to increase the frequency of the signal F.sub.OUT, which increased frequency causes charge to be pumped off of the first capacitor at an increased rate, wherein the amplifier forces the frequency of the F.sub.OUT signal to whatever rate is needed to maintain the charge on the first capacitor at essentially zero, whereby the frequency of the VCO output signal varies as a function of the magnitude of the analog current applied to the first capacitor.

In an implantable medical device, a low power current-to-frequency converter comprising:

• an amplifier having two input terminals and one output terminal, the amplifier comprising means for differentially amplifying an electrical signal applied between its two input terminals to provide an amplified output signal appearing on its output terminal;
• a first capacitor connected to one of the input terminals of the amplifier;
• a voltage controlled oscillator (VCO) circuit having a voltage-control input terminal and a VCO output terminal, the voltage-control input terminal being connected to the output terminal of the amplifier, the VCO including means for generating a VCO signal having a frequency that varies as a function of the magnitude of a voltage applied to the voltage-control input;
• a charge-pump circuit coupled to the first input terminal of the amplifier for pumping a discrete charge off of said first capacitor under control of the frequency of said VCO signal;
• wherein the electrical current is applied to the first capacitor and tends to cause a charge to accumulate on the first capacitor as a function of the magnitude of the electrical current, which charge tends to increase the output voltage of the amplifier so as to increase the frequency of the VCO signal, which increased VCO frequency causes the charge to be pumped off of the first capacitor at an increased rate, wherein the amplifier forces the frequency of the VCO signal to whatever rate is needed to maintain the charge on the first capacitor at essentially zero, whereby the frequency of the VCO signal varies as a function of the magnitude of the electrical current applied to the first capacitor.