GB2449273A - Reducing input switching noise in an amplifier - Google Patents
Reducing input switching noise in an amplifier Download PDFInfo
- Publication number
- GB2449273A GB2449273A GB0709396A GB0709396A GB2449273A GB 2449273 A GB2449273 A GB 2449273A GB 0709396 A GB0709396 A GB 0709396A GB 0709396 A GB0709396 A GB 0709396A GB 2449273 A GB2449273 A GB 2449273A
- Authority
- GB
- United Kingdom
- Prior art keywords
- noise
- impedance
- amplifier
- input
- input impedance
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- 230000000694 effects Effects 0.000 claims abstract description 5
- 239000003990 capacitor Substances 0.000 abstract description 19
- 238000000034 method Methods 0.000 abstract description 12
- 238000002593 electrical impedance tomography Methods 0.000 abstract description 2
- 230000003247 decreasing effect Effects 0.000 description 5
- 230000007423 decrease Effects 0.000 description 4
- 239000004065 semiconductor Substances 0.000 description 4
- 238000005259 measurement Methods 0.000 description 2
- 230000003287 optical effect Effects 0.000 description 2
- 235000014676 Phragmites communis Nutrition 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000003292 diminished effect Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000002347 injection Methods 0.000 description 1
- 239000007924 injection Substances 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03G—CONTROL OF AMPLIFICATION
- H03G3/00—Gain control in amplifiers or frequency changers
- H03G3/20—Automatic control
- H03G3/30—Automatic control in amplifiers having semiconductor devices
- H03G3/34—Muting amplifier when no signal is present or when only weak signals are present, or caused by the presence of noise signals, e.g. squelch systems
- H03G3/348—Muting in response to a mechanical action or to power supply variations, e.g. during tuning; Click removal circuits
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03G—CONTROL OF AMPLIFICATION
- H03G3/00—Gain control in amplifiers or frequency changers
- H03G3/20—Automatic control
- H03G3/30—Automatic control in amplifiers having semiconductor devices
- H03G3/34—Muting amplifier when no signal is present or when only weak signals are present, or caused by the presence of noise signals, e.g. squelch systems
- H03G3/345—Muting during a short period of time when noise pulses are detected, i.e. blanking
Landscapes
- Amplifiers (AREA)
Abstract
The invention diminishes the effects of noise in a non-inverting or inverting amplifier circuit by changing the effective input impedance of the amplifier during a switching event, e.g. during operation of an input selector or multiplexer. The effective input impedance is changed either directly by changing the value of components connected to the input or indirectly by altering connections to other elements, such as bootstrap capacitors, that determine the effective input impedance. The indirect method is superior as it uses switches connected to low impedance points so that the low impedances diminish the switching noise. The technique finds application in pad selector circuits for electrical impedance tomography.
Description
Removing Glitches in a High Impedance Sensor
Background
Noise can be introduced into the input of amplifiers by various circuit sources such as switches, multiplexers, and digital potentiometers by reception of external source from a sensor pad. For example CMOS logic 4066 type analogue semiconductor switches inject glitch noise into the signal line they change state. The amount of current injected per switch change is from about 0 1 pCoulomb to 1 pCoulomb. The more expensive switches reduce the charge injection by driving a small capacitor in the opposite direction to compensate for unbalance in the N and P type FET switches used. Switches using other technologies such as optical or magnetic also may introduce such noise when they change state. The noise may change the bias conditions or otherwise of the amplifier so that even after the noise has passed the amplifier output is not useable.
Statement of Invention
This invention diminishes the effects of noise in a non-inverting or inverting amplifier circuit by changing the effective input impedance. This change can either increase or decrease the input impedance at all frequencies or at selected frequencies or change the impedance parameters so that the noise content phase characteristics are changed. Said effective input impedance is changed either directly by changing the value of components connecting to the input or indirectly by altering other elements that are determining the effective input impedance. The indirect method shown is superior in the examples shown in that it uses switches connected to low impedance points so that the low impedances diminish the switch noise.
The direct method may inject noise as it is connected to high impedance points in the circuit. The indirect method alters elements of the circuit that are at low impedances and thus any noise injected by switches is diminished by the low impedances. The direct method is included here for completeness. The indirect method is generally preferred as it causes less noise when it switches.
Typically, the length of time the noise diminution circuit needs to be in effect is known from experience so that it only needs to be used for the minimum time.
Typically, the output of the amplifier is not measured at this time. Thus, for example if eight pads are being scanned through a multiplexer then the said noise diminution invention is used immediately before or at the same time that a different pad is selected and used for a short time before the output of the amplifier is measured.
The overall effect is to allow more accurate measurements to made and for the scanning of the eight pads to be faster.
Non-exclusive examples of directly changing the effective input impedance are: 1. For a non-inverting amplifier the effective input impedance can be made smaller by switching in a resistor in series with a capacitor connecting the input to OV or by changing the values of components, such as bias resistors, connected to the input.
2. For an inverting amplifier the effective input impedance can be made smaller by switching in a resistor in series with a capacitor connecting the input to OV or by changing the values of components, such as feedback resistors, connected to the input.
Non-exclusive examples of indirectly changing the effective input impedance are: 1. For a non-inverting amplifier the effective input impedance is often made larger by the use of bootstrapping. By removing totally or partially this bootstrapping, lower impedance is presented to the input 2. For an inverting amplifier the effective impedance of elements connected to the input is increased with decoupling elements. By removing totally or partially this decoupling, lower impedance is presented to the input.
The examples shown make the input impedance change resistively but this invention also covers means to switch in and out elements with band pass, low pass or high pass characteristics.
If semiconductor switches changing state are the generator of noise, we know when the noise will happen and the control line to the switch can be used as the point where said noise diminution invention is switched in. The invention covers the case where we do not know when the noise will happen and where we must use a noise sensing circuit based upon detection of input signals with certain rise time or frequency to provide a control for said noise diminution circuit.
Figures illustrating the invention Using a Non-inverting guard amplifier Figure 1 is an example of this invention applied to non-inverting guard amplifier of gain near one constructed with an operational amplifier 5 measuring a high impedance signal at a sensor pad. When switch 2 changes state noise current is injected into the input 10 causing the input to change in voltage and a glitch, elevation or depression of voltage, to appear at the output 11. This glitch will persist, as the injected current will have charged all capacitances connected to the input line such as the sensor pad to external world capacitance and the capacitance to the power supply lines of the switch 2.
An indirect method for decreasing input impedance and increasing it Disconnecting the bootstrapping capacitor 3 by opening a switch 4 makes the input impedance determined by the bias resistors 6, 7 and 8.
Connecting the bootstrapping capacitor 3 by closing switch 4 changes the effective input impedance typically over one hundred times greater than when it was disconnected. It does not introduce significant noise when switch 4 is closed as the injected nQise current from the switch goes into the low impedance junction point of 6 and 7. Optionally a resistor could be used in series with bootstrap capacitor 3 so that the bootstrapping of resistor 8 is partial when switch 4 closed.
A direct method for decreasing input impedance and increasing it We could decrease the input impedance by connecting with a switch 12 a resistor in series with a capacitor 9 in series to OV. When this switch was opened, current would be injected into the resistor in series with a capacitor 9 and into the output capacitance of the switch 12 causing output noise at 11.
Inverting amplifier Figure 2 is an example of this invention applied to an inverting amplifier, constructed with an operational amplifier, measuring a high impedance signal at a sensor pad 16. The noise source in this case is the series switch 17 connecting the sensor pad 16 with the input to the inverter 18.
Often this switch might be one out of a multiplexer tree. It uses a feedback element 20, that may be a capacitor for determining the gain of the circuit and a network consisting of a resistor 19 a decoupling capacitor 21 and resistor 22 from the output 26 to set up the DC bias conditions.
An indirect method for decreasing input impedance and increasing It Removing the connection of the decoupling capacitor 21 by opening a switch 26 decreases the input impedance since it makes the input impedance determined by the feedback series resistors 19 and 22.
Closing the switch 26, thus adding back into the circuit the decoupling capacitor 21, does not introduce noise as the injected current goes into the low impedance junction point of resistors 19 and 22 and the large capacitor 21. When switch 27 is closed, the input impedance from the bias network is typically over one hundred times greater as the AC feedback is decoupled. Optionally a resistor could be used in series with decoupling capacitor 21 so that the decoupling of resistor 22 is partial with switch 27 closed.
A direct method for decreasing input impedance and increasing it -.
We could decrease the input impedance by connecting with a switch 25 a resistor 23 in series with a capacitor 24 to OV. When this switch 25 was opened, current would be injected into the resistor 23 in series with a capacitor 24 and into the small output capacitance of the switch 25 causing a relatively large voltage on this capacitance which would drive the series resistor 23 capacitor 24 combination connected to the input 18 causing noise on the output 27.
We could instead use the switch 25 to connect the series resistor 23 and capacitor 24 arrangement to the output 27 decreasing the feedback during the presence of noise. However the switch 25 would inject noise when it was opened.
Application Information Electrical Impedance Tomography, Eli, is a technique applied to determine the distribution of impedance of a space. It generally uses contact sensors and six or more plates to cylindrically or spherically enclose the space so that the boundary conditions of the space are fixed. Conventionally one or more plates act as voltage or current transmitter and the remaining plates are scanned and used as a voltage or current receivers.
Typically the selection of which plates are used as transmitters or receivers or as a pad whose impedance is to be measured is done with semiconductor switches or a multiplexer, a set of semiconductor switches in single package with a common signal line. However, it can be done with reed or optical switches. These route the transmitter signals from the transmitting oscillators to the selected plates and the received signals to the receiver circuits and/or connect unselected pads to a isolating signal line that may be guard or CV.
Switching noise will be introduced when the above selecting switches are turned on and off. The noise reducing circuitry of this invention can be used at these points.
Also, single pad measurements of impedance to the external world can be used to determine contact impedance and/or to find the shape of an object. This cn be done with rnultiplexers choosing which pad to route to a single measurer of impedance or by having individual measurers of impedance for each pad. One example is the use of a capacitive array for determining through air the 3-D shape of an object. The impedances will be high and switching noise can be a problem when the selecting switches are turned on and off. The noise reducing circuitry of this invention can be used at these points.
Claims (5)
- Claims 1. A device comprising means to change circuit parameters sothat the input impedance of an amplifier is changed during the presence of noise in order to reduce noise effects.
- 2. A device according to claim 1 where a non-inverting amplifier is used and the input impedance of the amplifier is indirectly changed by switching in óôut bootstrapping of the bias network.
- 3. A device according to claim 1 where a non-inverting or inverting amplifier is used and the input impedance of the amplifier is directly changed by switching in or out impedances to the input.
- 4. A device according to claim 1 where an inverting amplifier is used and the input impedance of the amplifier is indirectly changed by switching in or out elements in the feedback impedance.
- 5. A device according to claim 1 with a means of determining the presence of noise to control the noise reducing circuitry.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0709396A GB2449273A (en) | 2007-05-15 | 2007-05-15 | Reducing input switching noise in an amplifier |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0709396A GB2449273A (en) | 2007-05-15 | 2007-05-15 | Reducing input switching noise in an amplifier |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB0709396D0 GB0709396D0 (en) | 2007-06-27 |
| GB2449273A true GB2449273A (en) | 2008-11-19 |
Family
ID=38234536
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0709396A Withdrawn GB2449273A (en) | 2007-05-15 | 2007-05-15 | Reducing input switching noise in an amplifier |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2449273A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9687169B2 (en) | 2011-12-08 | 2017-06-27 | Kimberly-Clark Worldwide, Inc. | System, controller, and method for determining conductance of an object |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002013380A2 (en) * | 2000-08-04 | 2002-02-14 | Maxim Integrated Products, Inc. | Methods and apparatus for muting the output of amplifier circuits |
| JP2002354571A (en) * | 2001-05-08 | 2002-12-06 | Lg Electronics Inc | Voice muting circuit |
| JP2003179435A (en) * | 2001-12-12 | 2003-06-27 | Fujitsu Ten Ltd | Mute circuit |
-
2007
- 2007-05-15 GB GB0709396A patent/GB2449273A/en not_active Withdrawn
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002013380A2 (en) * | 2000-08-04 | 2002-02-14 | Maxim Integrated Products, Inc. | Methods and apparatus for muting the output of amplifier circuits |
| JP2002354571A (en) * | 2001-05-08 | 2002-12-06 | Lg Electronics Inc | Voice muting circuit |
| JP2003179435A (en) * | 2001-12-12 | 2003-06-27 | Fujitsu Ten Ltd | Mute circuit |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US9687169B2 (en) | 2011-12-08 | 2017-06-27 | Kimberly-Clark Worldwide, Inc. | System, controller, and method for determining conductance of an object |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0709396D0 (en) | 2007-06-27 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |