US 5,606,320 A · Method and apparatus for micropower analog-to-digital conversion in an implantable medical device

Abstract

A micropower analog-to-digital converter (ADC) for use in an implantable medical device is disclosed. The ADC achieves a high conversion speed at micropower levels through a number of timing and circuit improvements over the conventional implementation of the successive approximation ADC architecture. The ADC includes a digital-to-analog converter (DAC) that preferably is implemented as a binary-weighted, switched capacitor array that employs top plate charging and performs bipolar conversion. The DAC provides an analog output signal representing array charge to a comparator. During a comparator latch phase, the DAC asynchronously determines a bit of the ADC digital output signal in response to the comparator output, and initiates a test of the next least significant bit during the same latch phase. Further, the DAC analog output signal is timed to settle during the latch phase in response to both the bit update and the next bit test. In conjunction with increasing conversion speed, the DAC reduces power requirements by using a power gated buffer that selectively increases the power of the ADC input signal only when necessary to drive the ADC conversion circuitry. Further, the comparator includes a precharged static memory cell to avoid crossover currents. Finally, the ADC includes a digital offset correction DAC that corrects not only the cumulative offset voltage generated by components of the ADC, but also the offset voltage of any device that provides the ADC input.

Claims26

Dependent claims

A micropower comparator for use in an implantable medical device, the comparator for receiving a first analog input signal and a second analog input signal, the comparator comprising:

a synchronous high gain amplifier for differencing the first and second analog input signals to thereby provide a balanced amplified difference signal during an acquisition phase, and for amplifying the difference signal to a logic level during a latch phase to thereby produce a differential latch output signal, wherein the differential latch output signal has a slew rate and the logic level has either a high logic level voltage or a low logic level voltage;
at least one precharged static memory cell (PSMC) for increasing the slew rate of the differential latch output signal without drawing crossover current, the at least one PSMC having a pair of differential outputs for providing a differential PSMC output signal, whereby current drain of the implantable medical device is substantially reduced;
and
a comparator detection logic circuit for providing a comparator logic level output upon detecting that the differential PSMC output signal has reached the logic level, the comparator logic level output representing the amplified difference of the first and second analog input signals.

1 more element

In a micropower successive approximation analog-to-digital converter (ADC) for converting an analog input signal into an N-bit digital output signal, the analog input signal representing a biophysical characteristic measured by an implantable medical device or an electrical characteristic of the implantable medical device itself, the ADC including a front end and a back end, the front end having sampling circuitry operating at a low frequency and at a low power for generating a sampled analog input signal, the back end having conversion circuitry operating at a high frequency and at a high power for determining the N-bit digital output signal that represents the sampled analog input signal, a power gated buffer comprising:

a switched buffer amplifier having an input for receiving the sampled analog input signal from the sampling circuitry and for providing a buffered output signal representing the sampled analog input signal;
and
a power gating switch for switching an operating power level of the switched buffer amplifier from a low operating level during a first phase to a high operating level during a second phase to thereby provide a power gated buffered output signal, wherein the power gated buffered output signal is provided during the second phase to the back end of the ADC before the back end begins determining a most significant bit of the digital output signal, whereby limiting the operating power level in this manner substantially reduces power requirements of the ADC.

A successive approximation, micropower analog-to-digital converter (ADC) for converting an analog input voltage, the ADC including sampling circuitry for generating a sampled analog input signal, the ADC comprising:

means for generating a reference voltage, a bias voltage, and a ground voltage;
a digital-to-analog converter (DAC) having a plurality of capacitors configured to produce a plurality of binary-weighted DAC analog output signals, each capacitor being coupled through a capacitor switch, wherein each capacitor switch position corresponds to respective bits of an ADC digital output signal, the DAC further including means for producing a plurality of net capacitor voltages corresponding to the difference between the sampled analog input signal and a respective one of the plurality of DAC analog output signals, each net capacitor voltage having a prescribed settling time associated therewith;
and
a comparator, having an acquisition phase and a latch phase, for determining the capacitor switch positions, the comparator including:
means for successively comparing each net capacitor voltage to the bias voltage during a respective acquisition phase to produce a difference signal;
conversion means for converting the difference signal to a logic level bit decision during a portion of a respective latch phase;
and
means for asychronously updating the ADC digital output signal and for asynchronously updating a next net capacitor voltage as soon as a bit decision results;
whereby the next net capacitor voltage starts to settle within a remaining portion of the latch phase;
whereby utilization of the remaining portion of the latch phase results in a reduction in required acquisition phase duration and a corresponding decrease in power requirements.

7 more elements

In a successive approximation, micropower analog-to-digital converter (ADC) for converting an analog input signal, the ADC including sampling circuitry for generating a sampled analog input signal, the ADC further including a digital-to-analog converter (DAC) and a comparator, the DAC including a plurality of binary-weighted capacitors configured in a parallel circuit arrangement, each capacitor being coupled through a switch to one of a reference or a ground voltage, the switch positions being determined by the comparator, wherein each switch position corresponds to respective bits of an ADC digital output signal, a method for converting the analog input signal, the method comprising the steps of:

controlling the capacitor switches to produce a plurality of binary-weighted DAC analog output signals;
producing a plurality of net capacitor voltages corresponding to the difference between the sampled analog input signal and a respective one of the plurality of DAC analog output signals, each net capacitor voltage having a prescribed settling time associated therewith;
determining, during an acquisition phase, a difference between a respective net capacitor voltage and a bias voltage using the comparator and producing a difference signal thereof;
converting, during a portion of a respective latch phase, the difference signal to a logic level bit decision;
asynchronously updating a bit of the ADC digital output signal as soon as a bit decision results;
asynchronously initiating, as soon as a bit decision results, a test of a next lower significant bit in the DAC to produce a next net capacitor voltage;
whereby the capacitors begin to settle within a remaining portion of the latch phase, thereby resulting in a reduction in required acquisition phase duration and a corresponding decrease in ADC power requirements.

4 more elements

Dependencies are inferred from the claim text.

Method and apparatus for micropower analog-to-digital conversion in an implantable medical device

Life of this patent

Method and apparatus for micropower analog-to-digital conversion in an implantable medical device

Life of this patent

Abstract

Claims26