Versatile detection circuit

Background

The present disclosure relates to an electric circuit for the detection of analog and of digital signals. More specifically, the present disclosure relates to an electric circuit for signal detection that is optimized for comparably low sensor voltages.

Circuits for the detection of analog and of digital signals are frequently based on an analog-to-digital (A/D) converter. An analog-to-digital converter changes an analog signal into a digital value. A/D converters are commonly employed to change a signal from a light sensor or from a temperature-dependent resistor into a digital signal. The digital signal can then be processed by a microprocessor unit or by any other digital circuit. Apart from analog signals, A/D converters may also be used to create a digital representation of a binary (on or off) input signal from a switch.

Especially in the field of measurements and instrumentation, microprocessor units frequently comprise built-in analog-to-digital converters. In other words, the microprocessor provides not only digital input pins, but also at least one analog input pin. The analog input pin is then connected to the internal A/D converter of the microprocessor unit.

Analog-to-digital converters come with given ranges of input voltages. In order to make best use of an A/D converter, the input signal to an A/D converter should ideally cover the entire range of input voltages of the converter. In order for the range of the sensor signal to match the range of the A/D converter, the sensor signal may have to be transformed. The comparably large voltage range their A/D converters has long been a shortcoming of microprocessors with built-in analog-to-digital converters. A large range of the input voltage of an A/D converter implies that a low-amplitude signal may cover only a part of the entire range of input voltages of the converter. A direct connection of a low-amplitude sensor signal to the wide-range input of an A/D converter then has the disadvantage of harnessing only a limited percentage of the input range of the converter. That disadvantage is often undesirable, especially when a certain digital resolution is required for the conversion of an analog signal into a digital signal. By way of example, a barometric pressure sensor will require a resolution of at least 12 bits corresponding to 4096 steps to detect also small changes in altitude. If the range of output voltages of the pressure sensor is limited to 0 ... 2 V, then a 12 bit A/D converter with an input range of 5 V will fail to deliver a true 12 bit digital representation of the pressure signal.

To date, practical solutions based on microprocessors with built-in analog-to-digital converters often require an external A/D converter to convert analog low-amplitude signals into digital signals with adequate resolutions. The digital output of the external analog-to-digital converter is then connected to the digital input of the microprocessor. The additional external A/D converter with a low range of input voltages adds to the complexity of the circuit and is yet another element prone to failure.

When an external A/D converter is not an option, an amplifier may be used to enlarge the voltage range of a sensor signal. The output of the amplifier is then connected to an analog input pin of the microprocessor with a built-in A/D converter. In this configuration, the amplifier needs to be highly linear in order to not impair the quality of the digital representation of the analog signal. The amplifier also comes with the disadvantages of the aforementioned external A/D converter.

The aim of the present disclosure is to at least mitigate the aforementioned difficulties and to provide detection circuits for digital and for analog signals that meet the aforementioned requirements.

Summary

The present disclosure is based on a discovery related to the advent of microprocessors with integrated A/D converters. This new class of microprocessor devices allows for the processing of low-voltage input signals with reduced complexity of the detection circuit.

It is an object of the present disclosure to provide a versatile detection circuit for low-amplitude electric signals.

It is a related object of the present disclosure to come up with a detection circuit that relies on a microprocessor with a built-in analog-to-digital converter. Advantageously, a low reference voltage may be chosen at corresponding pin of the microprocessor.

The above problems are resolved by a detection circuit for low-voltage signals according to the main claim of this disclosure. Preferred embodiments of the present disclosure are covered by the dependent claims.

It is another object of the present disclosure to provide a detection circuit that allows for the processing of low-voltage analog signals.

It is a related object of the present disclosure to provide a detection circuit that allows for the processing of low-voltage analog signals obtained from temperature-dependent resistors.

It is a related object of the present disclosure to provide a detection circuit that allows for the processing of low-voltage analog signals coming from positive thermal coefficient resistors.

It is a related object of the present disclosure to provide a detection circuit that is configured to process low-voltage analog signals coming from negative thermal coefficient resistors.

It is a related object of the present disclosure to provide a detection circuit for low-voltage analog signals that is configured to process both negative and positive input voltages.

It is another object of the present disclosure to provide a detection circuit that allows for the processing of low-voltage digital signals.

It is a related object of the present disclosure to come up with a detection circuit for low-voltage digital signals that allows for noise reduction and/or noise cancellation.

Brief description of the drawings

Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiments. The drawings that accompany the detailed description can be briefly described as follows:

• FIG 1 is a schematic providing an overview of the various steps of signal processing by a microprocessor embedded in a controller.
• FIG 2 is a special embodiment of the general concept of FIG 1. FIG 2 is a schematic of a detection circuit specially adapted to resistors up to 1.5 kOhm.
• FIG 3 is a special embodiment of the general concept of FIG 1. FIG 2 shows a detection circuit specially adapted to resistors up to 600 kOhm.
• FIG 4 is a special embodiment of the general concept of FIG 1. FIG 4 shows a detection circuit suited for the detection of the state of a (mechanical) switch.
• FIG 5 is a special embodiment of the general concept of FIG 1. FIG 5 is a detection circuit for the detection and for the measurement of voltages between -1 V and +11 V.

Detailed description

FIG 1 depicts a controller 1 with sensor 2 attached. The sensor 2 could, by way of example, be a photodiode, a temperature-dependent resistor, an acceleration sensor, a (barometric) pressure sensor or similar. The sensor 2 in the context of this application could also be a (mechanical) switch or a voltage source.

The controller 1 provides a current supply 3. The current supply 3 will be in use typically when the sensor 2 is a resistor or a switch. The current supply 3 may be switched off when the sensor 2 is a voltage supply.

The current supply 3 can be switched on and off through the microprocessor 4. To that end, the current supply 3 comes with a port 5b that is connected to a port 5a of the microprocessor 4. The microprocessor 4 is configured to send a (CSS) signal to the current supply 3 through the ports 5a, 5b in order to switch on or off the supply 3.

The current supply 3 is also connected to a power supply 6. In a preferred embodiment, the voltage supplied by the power supply 6 is +18 V or +19 V. In an advantageous embodiment, the magnitude of the current produced by the current supply 3 is proportional to the reference voltage. The dependence of the current from the current supply 3 on the reference voltage 10 helps mitigate the influence of variations in the reference voltage 10 on signal processing.

The power supply 6 receives a reference voltage 10 to vary the voltage supplied by the power supply 6 and the current supplied by element 3. The reference voltage would typically be 1.7 V. In another embodiment, the reference voltage is 2 V.

The reference voltage 10 also sets the maximum input range of the A/D converter 9. A low value of the reference voltage 10 reduces the current through and hence the stress on the sensor 2.

The sensor 2 is also connected to a low-pass filter 7. The low-pass filter 7 suppresses noise with frequencies above the mains frequency. In a preferred embodiment, the low-pass filter 7 provides a capacitor. This capacitor may prove useful whenever the sensor 2 is a switch. Upon closing the switch, the capacitor provides an extra current to help clean the contacts of the (mechanical) switch by removal of an oxide layer. The extra current supplied by the capacitor of the low-pass filter 7 may also reduce contact bounce.

In a particular embodiment, the low-pass filter 7 is a first-order RC or LC or RLC filter. In another embodiment, the low-pass filter is a Butterfly-type filter. In yet another embodiment, the low-pass filter is a Chebyshev-type filter. The latter two embodiments employ higher-order filters to better reduce 1/frequency noise that adds to the (weak) signal produced by a low-voltage sensor 2.

The controller 1 also provides a source 8 of alternating (AC) voltage to be added to the sensor signal. This source 8 of AC voltage introduces a technique known as oversampling. In a preferred embodiment, the frequency of the AC voltage is 270 Hz. The amplitude of the alternating voltage from the source 8 is typically about 15 mV. In a particular embodiment, the voltage supplied by the source 8 has triangular shape. In another embodiment, the source 8 essentially supplies a noise signal.

The controller 1 further provides an attenuation element 11. The attenuation element 11 is predominantly used together with sensors 2 that are -1 ... +11 V voltage sources, digital sensors, or high-impedance negative thermal resistance (NTC) sensors. In any of these cases, the voltage drop over the sensor 2 would exceed the maximum input voltage to be supplied to the A/D converter 9. Element 11 then reduces the amplitude of the signal. In a preferred embodiment, the reduction in amplitude is essentially proportional to the amplitude of the signal coming from the sensor 2. The attenuation element 11 provides an input port 12b. Through the input port 12b, the attenuation element 11 can receive a (MRS) signal from an output port 12a of the microprocessor 4. The microprocessor thus enables and disables the attenuation element 11 depending on whether attenuation is needed.

An impedance converter 13 is an optional element of the controller 1. The impedance converter 13 is required whenever due to (ambient) temperature dependence the input current of the A/D converter 9 becomes excessive. The impedance converter 13 then ensures current fed to the A/D converter 9 will suffice.

The analog-to-digital converter 9 preferably samples its input signal during 100 ms at a sampling rate of 2.5 kHz. In other words, the A/D converter 9 acquires 250 values. The values are then stored inside the microprocessor 4 for further processing. Through averaging a mean value with a higher resolution is obtained. With the AC signal added by element 8, averaging of the signal improves on the resolution of the A/D converter 9 through oversampling. That way, the resolution can be increased from 12 bits to 14 bits or even to 16 bits. The frequency of the AC voltage supplied by element 8 will be chosen accordingly and may actually be above 270 Hz.

In another embodiment, processing by the microprocessor 4 means detection of the mains frequency. The phase of the mains frequency may also be detected. The signal detected by the sensor 2 is often affected by the mains frequency. In addition, the mains frequency may influence the sensor 2 signal through electromagnetic interference. After sampling the signal during 100 ms, the microprocessor 4 detects the zero-crossing of the mains component of the signal frequency. The microprocessor 4 then determines the frequency at a resolution of preferably 0.1 Hz and/or also the phase of the mains component. The influence of the mains component may then be suppressed through a digital notch filter.

The controller preferably provides no element to amplify the voltage of the signal from the sensor 2. The arrangement is particularly advantageous, since it avoids sacrificing part of the resolution of the A/D converter 9. To attain this result, a microcontroller 4 is used with an integrated A/D converter 9 that relies on a comparably low reference voltage.

Now turning to FIG2, an exemplary embodiment with resistive sensors 102 is described. The detection circuit of FIG 2 has been adapted with particular emphasis on a resistive sensor 102 with resistance values of up to 1.5 kOhm. Typical examples of such sensors are Pt1000 or Ni1000 sensors.

In order to apply a voltage of 1.7 V to the resistive sensor 102, the supply voltage is lowered by a resistor 14. With the V_VCS being the supply voltage by element 6, R_sens being the resistance of the sensor, R₁₄ being the resistance of element 14, the voltage V_IN to ground at the input of the A/D converter 9 reads $${\mathrm{V}}_{\mathrm{IN}}={\mathrm{V}}_{\mathrm{VCS}}*{\mathrm{R}}_{\mathrm{sens}}/\left({\mathrm{R}}_{\mathrm{sens}}+{\mathrm{R}}_{\mathrm{14}}\right)\mathrm{.}$$

The resistor 705 and the capacitor 706 form the low-pass filter 7 described above in the context of FIG 1.

Now turning to FIG 3, a detection circuit specially adapted to resistive sensors 202 with resistance values of 0.1 kOhm to 600 kOhm is described. A negative thermal resistance sensor is a typical example in this category of sensors. The circuit also differs from the circuit of FIG 2 in that it comprises an attenuation element made up of the resistors 111 and 112. The attenuation element basically is a resistive voltage divider made up of the resistors 111 and 112. The same attenuation element is in principle known from element 11 of FIG 1.

With the symbols as described in the context of FIG 2 and with R₁₁₁ and with R₁₁₂ forming the attenuation element, the input V_IN to the A/D converter reads $${\mathrm{V}}_{\mathrm{IN}}={\mathrm{V}}_{\mathrm{VCS}}*{\mathrm{R}}_{\mathrm{sens}}//\left({\mathrm{R}}_{\mathrm{705}}+{\mathrm{R}}_{\mathrm{111}}+{\mathrm{R}}_{\mathrm{112}}\right)/\left({\mathrm{R}}_{\mathrm{sens}}//\left({\mathrm{R}}_{\mathrm{705}}+{\mathrm{R}}_{\mathrm{111}}+{\mathrm{R}}_{112}\right)+{\mathrm{R}}_{\mathrm{14}}\right)*{\mathrm{R}}_{112}/\left({\mathrm{R}}_{\mathrm{705}}+{\mathrm{R}}_{\mathrm{111}}+{\mathrm{R}}_{\mathrm{112}}\right)\mathrm{.}$$

The circuit as shown on FIG 4 is adapted for values of the sensor 202 resistance of less than 600 kOhm.

FIG 4 shows a detection circuit configured to receive a switch 302 as its sensor. The circuit of FIG 4 differs from the circuit of FIG 3 only in that the resistive sensor 202 has been replaced by the switch 202.

The capacitor 706 as shown in FIG 4 is not only part of the low-pass filter. It 706 also drives a current through the switch 302 upon closing of the (mechanical) contacts of the switch 302. The extra current from the capacitor 706 helps reduce contact bounce by clearing oxide layers on the surfaces of the contacts.

Now turning to FIG 5, a circuit for detecting and measuring voltages between -1 V and +11 V is presented. The predominant difference compared to the previous figures is that the sensor 2 has now been replaced by a voltage source 402. The voltage source 402 is connected to an power supply 406. Preferably, the power supply 406 delivers a voltage of -3 V. A resistor 414 together with the -3 V supply 406 allows for the detection of open circuits, when no resistor 414 is connected to the circuit. The power supply 406 and the resistor 414 are optional elements.

A low-pass filter made up of the elements 705 and 706 basically performs the same function as in the previous figures.

The resistors 411 and 412 form a resistive voltage divider. In a particular embodiment, the voltage V_IN at the input of the analog-to-digital converter attains a maximum of 1.1 V.

The voltage reference 410 separated from point 498 through a resistor 408. The resistor 409 increases the voltage at point 498 to approximately 0.1 V. The small increase in voltage at point 498 allows the circuit to also measure and to detect negative voltages at the sensor 402.

With the symbols as per the previous figures, with R₄₀₈ and R₄₀₉ being the resistance values of the resistors 408 and 409, with V₄₁₀ being the voltage at point 410, and with V₄₀₂ being the voltage of the sensor 402, the voltage V_IN reads $${\mathrm{V}}_{\mathrm{IN}}={\mathrm{V}}_{\mathrm{402}}*{\mathrm{R}}_{\mathrm{412}}/\left({\mathrm{R}}_{\mathrm{705}}+{\mathrm{R}}_{\mathrm{411}}+{\mathrm{R}}_{\mathrm{412}}\right)+{\mathrm{V}}_{\mathrm{410}}*({\mathrm{R}}_{\mathrm{409}}/\left({\mathrm{R}}_{\mathrm{408}}+{\mathrm{R}}_{\mathrm{409}}\right)*{\mathrm{R}}_{\mathrm{411}}/\left({\mathrm{R}}_{\mathrm{411}}+{\mathrm{R}}_{\mathrm{412}}\right)\mathrm{.}$$

With the exception of the impedance converter, the foregoing embodiments rely only on passive electric components for the transformation of the sensor signal. In other words, the circuits for the transformation of the sensor signal (2, 102, 202, 302, 402) comprise no amplifier to (proportionally) increase the voltage amplitude of the sensor (2, 102, 202, 302, 402) signal. Passive electric components comprise resistors, capacitors, and coils. Transistors or other semiconducting switches are not considered passive electric components.

The power supplies 6, 406, the current supply 3, the generator of the reference voltage 10, and the AC source (8) are not considered to be part of the transformation circuit. They (3, 6, 8, 10, 406) are instead supply components.

It should be understood that the foregoing relates only to certain embodiments of the invention and that numerous changes may be made therein without departing from the spirit and the scope of the invention as defined by the following claims. It should also be understood that the invention is not restricted to the illustrated embodiments and that various modifications can be made within the scope of the following claims.

Reference numerals

1

controller

2

sensor

3

current supply

4

microprocessor

5a, 5b

ports for switching on and off the current supply 3

6

power supply

7

low-pass filter

8

source of alternating voltage

9

analog-to-digital converter

10

voltage reference

11

attenuator

12a, 12b

ports for switching on and off the attenuator 11

13

impedance converter

14

resistor to define the voltage applied to the sensor

102

sensor with a resistivity of up to 1.5 kOhm

111, 112

resistors that form a resistive divider (attenuator)

202

sensor with a resistivity of up to 600 kOhm

302

(mechanical) switch

402

voltage source

406

source for the detection of open sensor contacts

408

resistor to separate the voltage reference 410 and point

498 409

resistor to raise the potential of point 498

410

voltage reference

411, 412

resistors that form a resistive divider (attenuator)

414

resistor for the detection of open sensor contacts

498

point with raised potential

499

input port of the analog-to-digital converter 9

705, 706

resistor and capacitor of the low-pass filter