Why to need fiducial voltage source?

This is an imitate world. No matter car, microwave oven still is a mobile phone, all electron equipment must be planted with some world of true ” of means and “ is alternant. For this, electronic equipment must can the measurement of will real world result (speed, pressure, length, temperature) the volume that map can measure in electronic world (voltage) . Of course, want to measure voltage, you need to measure a level. This standard is fiducial voltage. To systematic design personnel character, the problem does not depend on whether needing fiducial voltage source, use however why to plant fiducial voltage source?

 

 

Fiducial voltage source is a circuit or circuit element only, want circuit need only, it can offer foregone potential. If the product needs to gather the pertinent information of real world, for example size of batteries voltage or electric current, power comsumption, signal or character, breakdown identifying, must have relevant signal and a level comparative so. Every comparator, ADC, DAC or detect circuit must have ability of source of a fiducial voltage to complete afore-mentioned works (graph 1) . Have target signal and given value comparative, OK and accurate quantify any signal.

 

Graph 1: The typical usage of the fiducial voltage source of ADC

Norms of fiducial voltage source

Fiducial voltage source has a lot of forms and offer different character, but in the final analysis, precision and stability are the character with fiducial voltage the mainest source, because its main effect is,offer voltage of a foregone output. It is an error to the change at this given value. Norms of fiducial voltage source uses a figure normally the uncertainty that 2 definitions will come to forecast its to fall in certain condition.

 

Graph 2: Norms of source of high-powered fiducial voltage (part)

Initiative precision

Be in give temperature (it is 25°C normally) next measures change that output voltage. Although differ,the initiative output voltage of parts of an apparatus may differ, but if it to giving parts of an apparatus it is constant, so very easy its calibration.

 

Temperature drift

This norms is function of fiducial voltage source is evaluated use the most comprehensive standards, because it makes clear output,voltage follows the change of temperature. Temperature drift is mix by the blemish of circuit element nonlinear cause, accordingly often nonlinear.

 

To a lot of parts of an apparatus, temperature drift (TC – Temperature Coefficient, it is an unit with Ppm/°C) it is main error source. The parts of an apparatus with temperature consistent drift, it is OK of calibration. A about temperature drift common misunderstanding thinks it is linear. This brought about parts of an apparatus of such as “ to measure the point of view of and so on of can less ” in the drift inside lesser temperature limits, however fact often contrary. TC is appointed with ” of law of “ black box commonly, so that let a person,understand the likelihood inside limits of whole job temperature error. It is value of a computation, be based on the least value of voltage and maximum only, do not consider the temperature that these extreme value produce.

 

Appoint the fiducial voltage cause that first-rate degree of linearity has inside temperature limits to be in, or to those without the fiducial voltage source that adjusts carefully, can think error of the poorest condition and temperature limits become scale. Because,this is the biggest with the least output voltage is to be in likely extremely the biggest with what get below temperature of the least job. However, the fiducial voltage source that adjusts carefully to the course (normally through its low-down temperature drift decides) , its are nonlinear characteristic likelihood takes dominant place.

 

For example, appoint the apt of fiducial voltage source that is 100ppm/°C to there is quite good degree of linearity inside any temperature limits, the drift that because component does not match,causes masked his completely inherent nonlinear. Contrary, appoint the fiducial voltage source that is 5ppm/°C, its temperature drift will with nonlinear give priority to.

 

In the output tension that this shows 3 times in the graph and the concern with characteristic temperature see very easily. Attention, showed two kinds of likely temperature character among them. Not compensatory takes expression of source of unoccupied place fiducial voltage to be parabola, the least value is in place of temperature extreme value, maximum inter. The temperature compensation that shows here takes cause of unoccupied place fiducial voltage (like LT1019) expression is “S” form curve, its are the biggest the center that slope approachs temperature range. After a kind of circumstance falls, nonlinear aggravate, reduced the overall uncertainty inside temperature limits thereby.

 

Graph 3: Temperature of fiducial voltage source is characteristic
 

The optimal utility of temperature drift norms is computation appoints the biggest total error inside temperature limits. Unless understood temperature drift character very well, do not suggest computation did not appoint the error inside temperature limits commonly otherwise.

 

Long-term stability

This norms measures the trend that fiducial voltage changes along with time, have nothing to do with other variable. Initiative deflection basically is caused by the change of mechanical stress, latter originates normally down-lead frame, naked piece with moulding compound expand the difference of rate. Effect of this kind of stress often has very big initiative deflection, thereafter as time elapse, deflection can decrease quickly. Initiative drift still includes the change with electric and characteristic circuit element, include character of parts of an apparatus to go up in atomic level among them build. More long-term deflection is caused by the electric change of circuit element, often say for “ ageing ” . Compare with initiative drift photograph, apt of this kind of drift happens with inferior rate, and as time elapse changes rate can be reduced further. Accordingly, it often uses “ drift / √khr” will express. Below higher temperature, the ageing speed of fiducial voltage source often faster also.

 

This one norms often is ignored, but it becomes main error source possibly also. It is mechanical substantially, it is the result that hot loop brings about chip stress to change. After circulating through very big temperature, in give can observe below temperature arrearage, its behave the change to output voltage. It and temperature coefficient and time drift have nothing to do, can reduce the effectiveness of initiative voltage calibration.

 

During subsequently temperature circulates, apt of source of most fiducial voltage is in nominal change near output voltage, because this heats up arrearage normally with the maximum in the limit of that can forecast. Every manufacturer has him to appoint the method of this parameter, because this is typical,the value may produce misdirect. When estimation outputs voltage error, data manual (be like LT1790 and LTC6652) in offerred distributinging data will be more useful.

 

Other standards

The basis uses a requirement, the norms with other important possibility includes:

● voltage noise

● linear regulation / PSRR

● laden regulation

● presses difference

Limits of voltage of ● power source

● source current

 

Type of fiducial voltage source

Fiducial voltage source basically has two kinds: Billabong and series connection. Fiducial voltage source refers to series connection and billabong graph 4.

 

Graph 4: Fiducial voltage source
 

Billabong is fiducial voltage source

Billabong is fiducial voltage source is two upright parts of an apparatus, normally the design assigns the job inside voltaic limits to be in. Although most billabong is fiducial,voltage source is to take unoccupied place type to offer a variety of voltage, but can think they and neat accept diode use euqally easily, the fact is really such also.

 

A the commonnest circuit is source of will fiducial voltage join that cite a base reachs the ground, another brings a foot to join to resistor. And one of resistor brings a foot to join to resistor, another brings a foot to join to power source. Such, it turns a 3 end into circuit essentially. The public end of fiducial voltage source and resistor is output, and the public end of resistor voltage source and resistor also is output. The choice of resistor must proper, make inside limits of whole power source and load current limits, those who adopt fiducial voltage cause is the smallest it is with the biggest electric current inside rated limits. If power source voltage and load current change are not big, these fiducial voltage sources are used very easily at the design. If among them one of or both may produce major change, can bring about circuit normally actual dissipation power is compared nominal circumstance place needs to be gotten greatly much. Tell from this meaning, it can be thought to resemble A kind run like amplifier.

 

The advantage of fiducial voltage source includes billabong: The design is simple, enclose small, good stability is had below wide electric current and laden condition. In addition, it is designed very easily to lose fiducial voltage source, and can cooperate very tall power source voltage use (because exterior resistor is met share is major potential) , or cooperate voltage of low-down power source to use (because output is OK only under power source voltage a few millivolt) . Billabong product includes LT1004, LT1009, LT1389, LT1634, LM399 and LTZ1000. Typical billabong circuit is shown 5 times like the graph.

 

Graph 5: Billabong is fiducial voltage source
 

Series connection is fiducial voltage source

Series connection is fiducial voltage source is 3 (or more) upright parts of an apparatus. It more poor like low pressure (LDO) manostat, accordingly its a lot of advantages are identical. The most notable is, its use up relatively fixed source current inside range of very wide power source voltage, and be only when laden need just conduct load current. This makes its become power source voltage or load current to have the good choice of the circuit of bigger change. It is particularly useful in the circuit with very big load current, because there is series connection resistor between fiducial voltage source and power source.

 

Series connection product includes LT1460, LT1790, LT1461, LT1021, LT1236, LT1027, LTC6652, LT6660 to wait a moment. The product such as LT1021 and LT1019 can use as billabong or series connection are fiducial voltage source. Series connection is fiducial voltage source circuit is shown 6 times like the graph.