Offset Correction Apparatus for a Successive Approximation a/D Converter

The present invention relates broadly to an A/D converter correction apparatus, and in particular, to an offset correction apparatus for a successive approximation A/D converter.

In the prior art, the offset correction loop has been employed to maintain a constant, preset offset voltage that is referred to the input of the successive approximation analog to digital convertor apparatus. A half least significant bit (LSB) offset voltage is normally required (referred to the analog input of the A/D) to reduce quantizing errors at low level analog inputs. The offset correction circuit will correct for any low frequency, temperature related offsets internally generated in the successive approximation A/D converter apparatus. However, it will also correct for any low frequency offsets present at the analog input of the successive approximation A/D converter. The time constant of the correction loop is chosen to be long compared to the frequency of the analog signal being converted. This insures stable closed loop operation of the correction loop.

The conventional prior art offset correction circuit for successive approximation A/D converters rely on field effect transistor (FET) switches to break the closed loop operation of the A/D converter and sum in a correction voltage (V.sub.c) proportional to offsets either at the analog input or in the elements in the successive approximation A/D converter. The switching transients in the field effect transistor switches vary with temperature. These transients and the stray capacitance associated with the switches introduce an output offset voltage related to the duty cycle of the offset correction loop. In addition, any output offset voltage variation in the internal converter will not be corrected, since the FET switches will switch the internal D/A converter out of the loop during offset correction. In the prior art, the best offset correction that could be achieved over a 0.degree. C. to 70.degree. C. temperature range, is about .+-.10 mv DC when using the aforementioned conventional offset correction loop and standard A/D components. For an A/D converter whose least significant bit is less than 40 mV, a .+-.10 mV variation over temperature is not acceptable.

The present invention utilizes an offset correction circuit in a successive approximation A/D converter in which the internal D/A of the converter is incorporated into the offset correction loop. A timing circuit is used to provide a signal reaction loop. A timing circuit is used to provide a signal indicative of when the correction loop should be closed. When this signal occurs a digital zero is fed into a successive approximation register (SAR) and all bits out of the SAR remain at digital zero for the duration of the offset correction period. Thus, any non-zero voltage appearing at the output of the internal D/A of the converter will comprise an offset voltage which the offset correction circuit can correct.

It is one object of the present invention, therefore, to provide an improved offset correction apparatus utilizing the internal digital to analog conversion with the offset correction loop.

It is another object of the invention to provide an improved offset correction apparatus utilizing the timing signal associated with a digital to analog conversion to operate the correction circuitry.

It is still another object of the invention to provide an improved offset correction apparatus which will automatically compensate for a non-zero voltage at the output of the internal digital to analog converter.

These and other advantages, objects and features of the invention will become more apparent from the following description taken in conjunction with the illustrative embodiment in the accompanying drawings.

FIG. 1 is a block diagram of the offset correction apparatus in accordance with the present invention,

FIG. 2 is a signal timing diagram for the offset correction apparatus shown in FIG. 1, and

FIG. 3 is a block diagram of a test configuration of the offset correction apparatus to test offset correction operation.

Referring now to FIG. 1, there is shown an offset correction apparatus being utilized in a successive approximation A/D converter circuit to provide a correction for any offset voltage that exists or is generated within the converter circuit. The offset correction apparatus is shown in FIG. 1 as comprising an offset correction control circuit 10 and an offset correction voltage generating unit 12. The remaining circuitry from the analog input through to the digital output comprises the successive approximation A/D converter circuit. The converter circuit is comprised of a buffer 14 which receives the input analog signal and is connected to a sample and hold unit 16. The sample and hold unit 16 is connected through a second buffer 18 and a resistor 20 to a comparator unit 22. The output of the comparator unit 22 is applied through logic gate A which is part of the offset correction control circuit 10 to the successive approximation register 24. The internal D/A converter 26 and the amplifier 28 form a loop from the successive approximation register 24 to the comparator 22. The offset correction control circuit 10 comprises a timing unit 30 which receives a pre-trigger pulse and provides a pulse A and C to gates A and C respectively. Gate C which receives input data from the successive approximation register 24, provides an output signal to FET switch 32 and gate B. Gate B also receives a start conversion signal and in response to presence of both input signals, provides an output signal to the successive approximation register 24 to start the data conversion process. When the switch S1 in the offset correction voltage generating unit 12 is closed by FET switch 32, the integrating unit 34 is connected from the input of the comparator 22 to the positive input of the amplifier 28.

The new offset correction loop operates in the following manner. A system timing signal pre-trigger that indicates when the correction loop should be closed is applied to the timing unit 30. When the pre-trigger signal occurs, a digital zero is continually forced into the DATA input of the successive approximation register 24 for the duration of the correction time. Thus, the first successive approximation after the pre-trigger signal will set all bits out of the successive approximation register 24 to a digital zero (i.e., 0 volts out of the internal D/A). At the first conversion complete flag after the pre-trigger signal the start conversion pulse will be gated off by gate B. Thus, all bits out of the successive approximation register 24 will remain at a digital zero for the duration of the offset correction period. Since all bits out of the successive approximation register 24 are zero, any non zero voltage (V.sub.A/D) appearing at the internal D/A output will be an offset which will be corrected for by the loop. At the end of the offset correction period gate B will be enabled and normal operation of the successive approximation A/D will begin with the next start conversion pulse. There is shown in FIG. 2 the timing signals and their relationship to one another as they are used in the offset correction apparatus shown in FIG. 1 and FIG. 3.

Turning now to FIG. 3 there is shown the configuration of the offset correction apparatus that was utilized in the analysis of the operation of the offset correction apparatus. The successive approximation A/D Converter operates at a clock frequency of 10 MHz with a total conversion time of 1.2 micro seconds. The frequency of analog voltage being converted is approximately 400 KHz. The offset correction duty cycle is 5% with the offset voltage (referred to the analog input) for the converter set to +10 mVDC (1/2 LSB) by the offset reference. Values for resistors R1, R1, R3, R4 & C1 were chosen to give the offset correction loop an effective time constant of 2.0 seconds.

1. Integrator input offset voltage .+-.0.25 mV

2. Offset reference pot resistance variation .+-.0.06 mV

3. .+-.15 v regulation tolerance of .+-.1% .+-.0.05 mV

4. Comparator input offset voltage .+-.0.15 mV

Total Calculated Offset Error .+-.0.51 mV

The actual measured offset voltage (referred to the analog input) variation from a preset +10 mV (over a 0.degree. C. to 70.degree. C. temperature range) was shown to be less than .+-.1 mV. This type of offset correction loop may be utilized in a successive approximation A/D converter whose least significant bit is as small as 4.0 mV.

Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.