Abstract:
An offset correction circuit loop with summing nodes, a variable gain transconductance amplifier and capacitor. The input to the loop is sent to a first summing node and then to a separate circuit. The output of the separate circuit is sent to the output of the loop and to the input of a second summing node. The second summing node subtracts the circuit output from a reference voltage and sends the result to the transconductance amplifier which outputs a corrective current which is then integrated onto the capacitor to produce a corrective input offset voltage estimate.

Description:
FIELD 
     The present invention is related to an offset correction circuit. 
     BACKGROUND 
     A signal processing system may consist of one or more stages of amplification, filtering, buffering, or other operations on a voltage waveform. The voltage waveform is usually the sum of a time-varying signal, and a constant offset voltage which contains no information and may interfere with the processing of the voltage waveform. For example, the offset voltage may limit the amount of amplification that can be applied to the voltage waveform. 
     An offset correction loop can be used to remove a fixed offset or a slowly-varying offset. FIG. 1 shows a typical offset correction loop. The input voltage  100  is the sum of an offset voltage and a signal voltage. A clamp circuit  106  generates an estimate of the offset voltage which is subtracted from the input voltage  100  by a summer  110 . The offset voltage estimate is updated based on the difference between the output voltage  102  and the reference voltage  104  during offset calibrate intervals, during which the signal voltage is known to be zero. 
     For example, if the output voltage  102  is higher than the reference voltage  104  during an offset calibrate interval, the clamp  106  will increase the input offset estimate. If the output voltage  102  is lower than the reference voltage  104  during the offset calibrate interval, then the input offset estimate that is subtracted from the input voltage  100  is decreased. The loop settles when the output voltage  102  equals the reference voltage  104  during the offset calibrate intervals. 
     Normally, circuits not only take offset voltages from input signal waveforms but also have internal offsets which create offset voltages at their outputs. For example, to remove the offset of an amplifier, one could use an offset correction loop prior to the amplifier to keep it from saturating as a result of input offset voltage, and another offset correction loop after the amplifier to remove any offset voltage produced by the amplifier itself. The offset correction loop of FIG. 1 can be used for both purposes. 
     In FIG. 2, a offset correction circuit  200  is located between the output of the first summer  110  and the output voltage  102 . The advantage of this architecture is that a single offset correction loop can be used to remove offsets in both the input waveform and the added circuit. 
     SUMMARY 
     The present invention is directed to an offset correction circuit comprising a first summing unit, a second summing unit, a differential transconductance amplifier, and a capacitor. The differential transconductance amplifier is located between the output of the second summing unit and the second input of the first summing unit; the capacitor located between the second input of the first summing unit and a first reference voltage. The offset correction circuit input may be located before the first input of the first summing unit and the offset correction circuit output may be located between the output of the first summing unit and the first input of the second summing unit. In other embodiments, the differential transconductance amplifier may be implemented with fully differential output and may use variable resistors. The variable resistors may be implemented with NMOS devices. In another embodiment the differential transconductance amplifier is nonlinear. 
     In an embodiment of the invention a separate circuit is located between the output of the first summing unit and the circuit output. In another embodiment a switch is located between an output of variable gain amplifier and the capacitor. A second reference voltage may be coupled to the second input of the second summing unit. In a further embodiment a lowpass filter is located between the output of the second summing unit and an input of the voltage integrator. The lowpass filter may be an R-C filter employing a switched-capacitor resistor. 
     A further embodiment of the invention is directed to a method of correcting a circuit offset, the method comprising the steps of: providing a voltage integrator, comprising an transconductance amplifier and a capacitor, between a circuit input and a circuit output; calibrating the voltage integrator by storing an input offset voltage on the capacitor; and subtracting the input offset voltage estimate from an input waveform. 
     A further embodiment of the invention is directed to an offset correction loop comprising a transconductance amplifier connected between a circuit input and a circuit output, a capacitor connected between a circuit input and a reference voltage, and a switch controlled by an offset calibrate signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an offset correction loop according to the prior art; 
     FIG. 2 shows an offset correction loop around a circuit according to the prior art; 
     FIG. 3 shows an offset correction loop with a voltage integrator according to the invention; 
     FIG. 4 shows an offset correction loop with a voltage integrator around a circuit according to the invention; 
     FIG. 5 shows an offset correction loop with a lowpass filter and voltage integrator around a circuit according to the invention; 
     FIG. 6 shows a transistor level diagram of a conventional transconductance circuit according to the prior art; 
     FIG. 7 shows a detailed diagram of a variable gain transconductance circuit with variable resistors and single output according to the invention; 
     FIG. 8 shows a detailed diagram of a variable gain transconductance circuit with triode region NMOS load and differential output according to the invention; 
     FIG. 9 shows the transfer characteristic of a variable gain transconductance circuit according to the invention; 
     FIG. 10 shows a lowpass filter according to the prior art; 
     FIG. 11 shows a switched-capacitor lowpass filter according to the prior art; 
    
    
     DETAILED DESCRIPTION 
     The offset correction loops of FIGS. 1 and 2 can be approximated as linear first-order systems during offset calibration intervals. Therefore, their start-up behavior and response to changes in the input offset voltage are determined by a clamp loop time constant, as well as by the time between and duration of the offset calibration intervals. System constraints often fix the time between calibration intervals and the duration of those intervals, but the clamp loop time constant can be adjusted to trade-off start-up time and noise in the offset estimate. 
     In the preferred embodiment, the clamp circuit  106  is a voltage integrator, consisting of a transconductance stage and a capacitor. FIG. 3 shows this embodiment of the offset correction loop. The clamp block is shown as a transconductance stage  300 , a switch  302  controlled by the offset calibrate signal, and a capacitor  304 . After an initial calibration, when the offset calibration signal is active, the switch  302  is opened and the constant input offset voltage estimate held on the capacitor  304  is subtracted from the input waveform  100 . When the offset calibration signal is active (during an offset calibration interval), the switch  302  is closed. The transconductance stage  300  delivers a current which is a positive function of its input voltage. Its input voltage is a function of the reference voltage  104  subtracted from the output voltage  102 . The current delivered by the output of transconductance stage  300  is integrated onto the capacitor  304 . If the output voltage  102  exceeds the reference voltage  104 , the transconductance current charges up the capacitor, increasing the voltage subtracted from the input waveform. If the reference voltage  104  exceeds the output voltage  102 , the capacitor  304  is charged down, decreasing the voltage subtracted from the input waveform  100 . 
     FIG. 4 shows the offset correction loop of FIG. 3 with a circuit  400  placed between the output of the first summer  110  and the output voltage  102 . This loop operates in a similar manner to the loop of FIG. 2 where the offset voltage from the input waveform  100  and the offset voltage created by the circuit  400  are both removed by the offset correction of FIG.  4 . 
     The time constant of a linear correction loop is a measure of the amount of time needed by the offset correction loop to settle, meaning the amount of time necessary to compensate for any offset voltage. The time constant must be low enough so that the correction loop can correct for offsets which change in time, but high enough that the input offset estimate is not too noisy. For example, in a CCD imaging system, a noisy input offset estimate appears as horizontal streaks in the image. The start-up time (the time required for the loop to settle when the power is turned on) is limited by the maximum output current of the transconductance stage. For a linear transconductance, this time is proportional to the loop time constant, forcing a trade-off between start-up time and noise in the input offset estimate. 
     FIG. 6 shows a detailed diagram of a conventional transconductance circuit, in which the output current is approximately proportional to the input voltage. FIG. 7 shows a variable gain transconductance amplifier which is used in an embodiment of the invention. The input transistors  701  and  702  convert the input voltage V in  to a differential current which is then converted back into a differential voltage V(n 1 , n 2 ) by the variable resistors  710  and  711 . This differential voltage is converted back into a differential current through NMOS transistors  703  and  704  and PMOS transistors  705  and  706 . The overall gain, I out /V in , can be changed through the variable resistance. A common mode feedback circuit  720  is required to bias the circuit correctly, but does not directly affect the gain. 
     FIG. 8 shows a specific implementation of the variable gain transconductance stage shown in FIG. 7 where the variable resistors are implemented by NMOS transistors. NMOS transistors  801  and  802  are provided a fixed gate bias voltage vbias which keeps the NMOS devices  801  and  802  in the triode region for small input voltages V in . In the triode region of operation, transistors behave like resistors for small drain-source voltages, where the effective resistance is set by the gate-source voltage. As V in  increases, more current flows through the PMOS transistor  803  and NMOS  801 , increasing transistor  801 &#39;s drain voltage n 1 . As n 1  increases beyond a certain voltage, transistor  801  moves into the saturated region of operation and the drain impedance greatly increases. When this occurs, the voltage gain V(n 1 , n 2 )/V in  greatly increases and the overall transconductance greatly increases. Furthermore, FIG. 8 shows the transconductance stage with fully differential output, which is convenient for systems in which the signal path is itself fully differential. Differential signal paths are commonly used in analog circuitry to reduce the influence of power supply noise or noise coupled in from other circuitry. A further benefit of differential signal paths is a doubling of signal range, which can improve signal-to-noise ratio. 
     FIG. 9 shows a typical V in  versus I out  transfer characteristic for the transconductance amplifiers of FIGS. 7 and 8. For small V in , the slope of the characteristic is approximately constant and the circuit behaves like a conventional linear transconductance amplifier. For large V in , the output current is much higher. The advantage of this transfer characteristic is that it simultaneously allows a short start-up time and a long loop time constant. The potential disadvantage of the nonlinear transfer function is that large error spikes in the input waveform produce a change in the input offset estimate out of proportion to their magnitude, making the input offset estimate appear noisy. Using a lowpass filter as shown in FIG. 5 reduces this effect by smoothing out the spike so that the transconductance amplifier&#39;s input voltage stays within the low-gain region. 
     FIG. 10 shows a basic lowpass filter. Using this lowpass filter in the offset correction circuit of the invention has two specific problems. First, the time constant of the lowpass filter is determined by the product of a resistance and a capacitance, which results in process and temperature sensitivities. Second, the capacitive load presented to the circuit which drives the lowpass filter is the sum of the lowpass filter capacitance and the transconductance amplifier input capacitance, which can be a large quantity, which makes it difficult to drive. 
     FIG. 11 shows an improved version of the lowpass filter which utilizes a two phase non-overlapping clock. C p    1208  represents the input capacitance of the transconductance stage and is not a distinct capacitor itself. The time constant of this lowpass filter is set by the ratio C 1 /C p  of the capacitances of the capacitors  1206  and  1208 , and the clocking frequency of the switches  1202  and  1204 , all of which can be well controlled. A further advantage is that capacitor  1206  can be substantially smaller than the input capacitance of the transconductance stage  1208  so that the circuit which drives the lowpass filter need only have enough power to drive capacitor  1206 , and not the transconductance amplifier input capacitor. This basic input configuration is referred to as a switched-capacitor resistor.