Chopper-stabilized amplifier and magnetic field sensor

Not Applicable.

Not Applicable.

This invention relates generally to chopper-stabilized amplifiers and to magnetic field sensors that use chopper-stabilized amplifiers and, more particularly, to a chopper-stabilized amplifier that can reduce an offset component of an input signal and provide improved common mode rejection while supporting a relatively high switching frequency, and to a magnetic field sensor that uses such a type of chopper-stabilized amplifier.

As is known, some types of analog electronic signals include both a signal component and an offset component. The offset component is usually an undesirable DC component, while the signal component can be an AC component that usually carries information used by an electronic system that receives the electronic signal. It will be recognized that, if such an electronic signal is received by and amplified by an amplifier, the amplified offset component might saturate the amplifier or cause other undesirable effects upon the electronic system receiving the electronic signal. Also, an amplifier itself has an input offset voltage, which, if greatly amplified, can saturate the amplifier or another amplifier or cause undesirable effects upon the system.

Chopper-stabilized amplifiers are used to reduce the effects of offset components of signals. Some chopper-stabilized amplifiers receive an input signal having a signal component and an offset component and operate to reduce the offset component relative to the signal component. Chopper-stabilized amplifiers having a variety of configurations are known.

Magnetic field sensors are used in a wide variety of applications. As one example, magnetic field sensors are widely used in automobiles for mechanical position sensing. For example, one type of magnetic field sensor is used as a rotation sensor in antilock brake systems.

Some types of magnetic field sensors use a Hall effect element, sometimes referred to as a Hall plate. A Hall effect element can generate a signal with both a signal component and an offset component, which can be due to resistance gradients, geometrical asymmetries, and piezoresistive effects. The magnitude and polarity of the offset component (or offset voltage) can be related to stresses in a semiconductor substrate supporting the Hall effect element, which stresses tend to vary with mechanical pressure and with temperature. Various techniques have been used to address and cancel the Hall effect element offset voltage, including use of a chopper-stabilized amplifier in combination with a current spinning technique, which is described more fully below.

One type of Hall effect sensor includes a switched Hall element, a chopper-stabilized amplifier, and a low pass filter. The switched Hall element, sometimes referred to as a spinning Hall element, includes both a Hall element having (typically) four contacts and a modulation circuit to periodically connect a supply voltage and an amplifier input to one pair of contacts or the other. Quadrature phases of operation are provided by complementary (180 degree out of phase) clock signals. Operation of a switched Hall element is described more fully below in conjunction with FIGS. 2-3C.

Use of a switched Hall element provides a way to discriminate (and relatively reduce) the Hall effect element offset voltage (referred to herein as an offset component) from the magnetically induced signal (referred to herein as a magnetic field signal component). In one type of modulation circuit used in a switched Halle element circuit, the modulation circuit modulates the magnetic signal component and the offset component remains substantially unchanged, i.e., un-modulated. In conjunction with this modulation circuit, a chopper-stabilized amplifier demodulates the magnetic signal component and modulates the offset component. The offset component, which, after the chopper-stabilized amplifier is an AC signal, is attenuated by a low pass filter to provide the magnetic field sensor output signal without an offset component. The technique effectively removes the Hall effect element offset voltage.

It will be appreciated that it is desirable to make a switching frequency used in the chopper-stabilized amplifier as high as possible. A higher switching frequency tends to result in an ability to use a low pass filter with a wider passband, which further tends to result in the magnetic field sensor having a faster response time.

It will also be understood that the magnitude of any ripple remaining on the magnetic field sensor output signal must be balanced with the sensor response time. As the low pass filter passband is reduced to reduce the ripple, the response time of the magnetic field sensor increases, which is undesirable.

Conventional chopper-stabilized amplifiers and magnetic field sensors using the chopper-stabilized amplifiers suffer from certain disadvantages. For example, some conventional chopper-stabilized amplifiers require a relatively low switching frequency, and therefore, as described above, a low pass filter with a relatively narrow passband and a resulting relatively slow response time. For another example, some conventional chopper-stabilized amplifiers suffer from having a relatively low common mode rejection and/or a relatively low rejection of the offset component.

It would be desirable to provide a chopper-stabilized amplifier that can accommodate a relatively high switching frequency, that has a relatively high rejection of a common mode signal, and that has a relatively high rejection of an offset component of an input signal.

In some embodiments, a magnetic field sensor includes a Hall element, a modulation circuit, a chopper-stabilized amplifier, and a filter, all coupled in series. The output signal from the Hall effect element can include a magnetic field signal component and an offset component. The modulation circuit is responsive to the output signal of the Hall element and operates to modulate only the magnetic field signal component of the output signal from the Hall element. The chopper-stabilized amplifier is responsive to the modulation circuit output signal and provides an amplifier output signal with a reduced offset component. The chopper-stabilized amplifier has particular characteristics such that the chopper-stabilized amplifier can accommodate a relatively high switching frequency, has a relatively high rejection of a common mode signal, and has a relatively high rejection of an offset component of an input signal.

In accordance with one aspect of the present invention, a chopper-stabilized amplifier includes first and second front-end amplifiers, each having a respective input node and a respective feedback node. The first front-end amplifier is configured to generate a first amplified output signal and the second front-end amplifier is configured to generate a second amplified output signal. The chopper-stabilized amplifier also includes a first switching network coupled to receive a binary clock signal having first and second periodic states, coupled to receive the first and second amplified output signals, and configured to generate first and second switched output signals. During the first state of the binary clock signal, the first switched output signal is related to the first amplified output signal and the second switched output signal is related to the second amplified output signal. During the second state of the binary clock signal, the first switched output signal is related to the second amplified output signal and the second switched output signal is related to the first amplified output signal. The chopper-stabilized amplifier also includes first and second back-end amplifiers. The first back-end amplifier is coupled to receive the first switched output signal and the second back-end amplifier is coupled to receive the second switched output signal. The first back-end amplifier is configured to generate a first back-end output signal and the second back-end amplifier is configured to generate a second back-end output signal. An output signal of the chopper-stabilized amplifier is a differential signal comprised of the first and second back-end output signals. The chopper-stabilized amplifier also includes at least one of a feedback circuit or a second switching network coupled to receive the first and second back-end output signals. The other one of the feedback circuit or the second switching network is configured to generate first and second switched feedback signals. The second switching network is coupled to receive the binary clock signal. During the first state of the binary clock signal, the first switched feedback signal is related to the first back-end output signal and the second switched feedback signal is related to the second back-end output signal. During the second state of the binary clock signal, the first switched feedback signal is related to the second back-end output signal and the second switched feedback signal is related to the first back-end output signal. The feedback node of the first front-end amplifier is coupled to receive the first switched feedback signal, and the feedback node of the second front-end amplifier is coupled to receive the second switched feedback signal.

In accordance with another aspect of the present invention, a magnetic field sensor includes a Hall effect element having two output nodes at which is provided a differential output signal that varies in response to a magnetic field. The Hall effect element differential output signal includes a magnetic field signal component and an offset component. The magnetic field sensor also includes a chopper-stabilized amplifier as described above. The input nodes of the first and second front-end amplifiers are coupled to the Hall effect element.

In some embodiments, the magnetic field sensor also includes a modulation circuit coupled between the Hall effect element and the chopper-stabilized amplifier and coupled to receive the binary clock signal. The modulation circuit is operable to modulate the magnetic field signal component to provide to the first and second front-end amplifiers a modulated signal having the modulated magnetic field signal component and having an un-modulated offset component.