CN105591646A - RS-485 input common-mode range expansion circuit adaptive to power supply voltage - Google Patents

Abstract

The invention discloses an RS-485 input common-mode range expansion circuit adaptive to power supply voltage. The RS-485 input common-mode range expansion circuit comprises a bias circuit, a buffer amplifier, an input voltage regulating circuit and a potential translation circuit. The bias circuit comprises a first resistor and a second resistor. The common end of the first resistor and the second resistor is connected with the input end of the buffer amplifier. The input voltage regulating circuit comprises a third resistor and a fourth resistor. The third resistor and the fourth resistor are connected in series. One end of the series circuit is a voltage input port, and the other end is connected with the output end of the buffer amplifier. The potential translation circuit comprises a first triode and a second triode. A base electrode is connected with the common end of the third resistor and the fourth resistor. The RS-485 input common-mode range expansion circuit has a simple structure and furthermore is adaptive for the voltage change of a power supply, so that the device at the input port can operate normally on a low power supply voltage condition, and a higher comparison precision can be realized on a high power supply voltage condition.

Description

The RS-485 input common-mode range expanded circuit of adaptive supply voltage

Technical field

The present invention relates to common mode range expanded circuit field, be specifically related to a kind of adaptive power supply electricityThe RS-485 input common-mode range expanded circuit of pressing.

Background technology

RS-485 bus communication technology is because its wire laying mode is easy, and communication distance is long, compatibilityThere is the feature such as polarity and nonpolarity connection, at present in ammeter, water meter, gas meter and Industry ControlField is widely used. Slave in the time of group-net communication in RS-485 bus by testbus onDifferential voltage come Receiving Host send signal. But because applied environment is complicated and changeable, in additionCommunication distance is longer, and for cost consideration, between node and node, conventionally can not adoptDesign altogether, so just require RS-485 communication interface chip can adapt to wider common mode defeatedEnter scope, the common-mode input range of General Requirements RS-485 interface circuit is (7V, 12V).Be directly inputted to reception comparator input terminal at signal of wide common mode range so, be easy to make defeatedThe device of inbound port is because of saturated or cut-off cisco unity malfunction. Especially at low voltage power supplySituation under, the operation window of device reduces, and causes this problem more serious.

Summary of the invention

For the deficiencies in the prior art, the invention discloses a kind of adaptive supply voltageRS-485 input common-mode range expanded circuit.

Technical scheme of the present invention is as follows:

A RS-485 input common-mode range expanded circuit for adaptive supply voltage, comprises partiallyCircuits, buffer amplifier, input voltage regulation circuit and current potential translation circuit;

Described biasing circuit comprises the first resistance and the second resistance, described the first resistance and the second electricityResistance series connection, this series circuit one end connects supply voltage, other end ground connection; Described the first resistanceBe connected the input of buffer amplifier with the common port of the second resistance;

Described input voltage regulation circuit comprises the 3rd resistance and the 4th resistance; Described the 3rd resistanceWith the 4th resistance series connection, this series circuit one end is voltage input end mouth, and the other end connects bufferingThe output of amplifier;

Described current potential translation circuit comprises the first triode and the second triode, described the one or three utmost pointPipe and the second triode are PNN type triode, the current collection of the first triode and the second triodeUtmost point ground connection, base stage connects the common port of the 3rd resistance and the 4th resistance, and emitter stage is as circuitOutput, the input of comparator circuit in connection late-class circuit.

Its further technical scheme is: described the 3rd resistance is variable resistor; Also comprise detection-control module, described detection-control module comprises draws together the 5th resistance, the 6th resistance, comparatorAnd N-type metal-oxide-semiconductor; Described the 5th resistance and the 6th resistance, this series circuit one end connects electricitySource voltage, other end ground connection; The 5th resistance is just being connected comparator with the common port of the 6th resistanceTo input, the negative input input reference voltage of comparator, the output of comparator connectsThe grid of metal-oxide-semiconductor, the drain electrode of metal-oxide-semiconductor connects one end of the 3rd resistance, and the source electrode of metal-oxide-semiconductor connectsConnect the resistance adjustable side of the 3rd resistance.

Useful technique effect of the present invention is:

The present invention proposes a kind of simple in structure, can follow mains voltage variations and automatically adapt toCommon mode amplifying circuit, even if make the device of input port under low supply voltage, also can be justNormal work, and can provide higher comparison precision high power supply voltage is next.

Brief description of the drawings

Fig. 1 is circuit diagram of the present invention.

Fig. 2 is the principle schematic of detection-control module.

Detailed description of the invention

Fig. 1 is circuit diagram of the present invention. The present invention includes biasing circuit, buffer amplifier, defeatedEnter voltage regulator circuit and current potential translation circuit.

Biasing circuit comprises the first resistance R 1 and the second resistance R 2, the first resistance R 1 and secondResistance R 2 is connected, and one end of this series circuit connects supply voltage VDD, one end ground connection. TheOne resistance R 1 is connected the input of buffer amplifier Buffer with the common port of the second resistance R 2.

Input voltage regulation circuit comprises the 3rd resistance R 3 and the 4th resistance R 4. The 3rd resistance R 3Connect with the 4th resistance R 4, this series circuit one end is voltage input end mouth PINA, anotherEnd connects the output of buffer amplifier Buffer.

Current potential translation circuit comprises the first triode Q1 and the second triode Q2. The first triodeQ1 and the second triode Q2 are PNN type triode, the first triode Q1 and the two or three utmost pointThe grounded collector of pipe Q2, base stage connects the common port of the 3rd resistance R 3 and the 4th resistance R 4.The emitter stage of the first triode Q1 and the second triode Q2 is defeated as circuit of the present inventionGo out end, connect late-class circuit, the i.e. input port of comparator circuit.

As shown in Figure 1, operation principle of the present invention is:

The output voltage V cm of buffer amplifier Buffer is:

$Vcm=VDD*\frac{R2}{R1+R2}=K1*VDD---\left(1\right)$

Buffer amplifier Buffer is by the first resistance R 1 in biasing circuit and the second resistance R 2The output voltage signal of common port strengthens.

The voltage of voltage input end mouth PINA place input is VA; And the first triode Q1 andThe base voltage Vb of the second triode Q2 is:

$Vb=K1*VDD+(VA-K1*VDD)*\frac{R4}{R4+R3}---\left(2\right)$

By in formula (2)Be designated as K2;

Can be obtained by formula (1), formula (2):

$\begin{array}{c}Vb=K1*VDD+(VA-K1*VDD)*K2\\ =K1*(1-K2)*VDD+K2*VA\end{array}---\left(3\right)$

The bar that can normally work due to the first triode Q1 and the second triode Q2 of PNN typePart is base voltage scope between-0.3V～VDD-0.7V, if base voltage lower than-0.3V,May there is positively biased electric leakage in the BC of triode knot, if base voltage is higher than VDD-0.7V,The BE knot of triode cannot normal bias. There is the boundary condition of common mode input:

-0.3≤Vb≤VDD-0.7V(4)

Bring formula (3) into boundary condition shown in formula (4):

K1*(1-K2)*VDD+0.3≥-K2*VA(5)

[1-K1*(1-K2)]*VDD-0.7V≥K2*VA(6)

Because being, K1 and K2 be greater than 0 and be less than 1 proportionality coefficient, therefore in formula (5)And in formula (6), the coefficient of supply voltage VDD is all for just, therefore by formula (5) and formula (6)Easily learn, for same VA, K2, K1, the value of VDD is larger, more easily meets common modeThe boundary condition of input; Equally, for same VA, K1, VDD, K2 more approaches 0, moreEasily meet the boundary condition of common mode input.

But for high-precision application, K2 is less, the error that circuit is corresponding is larger, supposesThe random imbalance of buffer amplifier Buffer and the error that unbalance of system causes are V_OS, right with itShould, the error of inputting on voltage input end mouth PINA will be exaggerated, and multiplication factor is:

$V_{OS\_PINA}=V_{OS}*\frac{1}{K2}---\left(7\right)$

Therefore, for precision is improved, K2 should meet the boundary condition of common mode inputUnder prerequisite, get higher value as far as possible.

And supply voltage VDD varies in size, the range of choice of K2 is also different.

In the time that supply voltage VDD is larger, the value of K2 is larger, as works as VDD=5V, K1=0.3,When K2=0.2, the input voltage VA of voltage input end mouth PINA can be at (7V ,+12V)In scope, change.

But in the time that supply voltage VDD is lower, the range of choice of K2 diminishes, as VDD=3.3V,K1=0.3, now when K2=0.1, could be at the input voltage of voltage input end mouth PINAWhen VA changes in (7V ,+12V) scope, meet common mode input shown in formula (4)Boundary condition. And convolution (7), 0.1 consequence of bringing that is contracted to of K2 is that error increases100％。

Therefore, the invention allows for more excellent technical scheme, increase detection-control moduleDet-Control, effect is to detect supply voltage VDD, and regulates R3's according to testing resultSize is to reach the object that regulates K2. In the time that VDD is higher, reduce R3, increase K2 to carryHigh relatively precision; In the time that VDD is lower, increase R3, reduce K2, to meet common mode inputBoundary condition.

Fig. 2 is the principle schematic of detection-control module. Detection-control moduleDet-Control comprises the 5th resistance R 5, the 6th resistance R 6, comparator C OMP and N-typeMetal-oxide-semiconductor M1, and the 3rd resistance R 3 is variable resistor. The 5th resistance R 5 and the 6th resistance R 6Series connection, one end of this series circuit connects supply voltage VDD, other end ground connection. The 5th resistanceR5 is connected the positive input of comparator C OMP, comparator with the common port of the 6th resistance R 6The negative input input reference voltage Vref of COMP, the output of comparator C OMP connectsThe grid of metal-oxide-semiconductor M1, the drain electrode of metal-oxide-semiconductor M1 connects one end of the 3rd resistance R 3, metal-oxide-semiconductorThe source electrode of M1 connects the resistance adjustable side of the 3rd resistance R 3. Metal-oxide-semiconductor M1 is equivalent to a switchCircuit.

As shown in Figure 2, the 5th resistance R 5 and the 6th resistance R 6 are sampled to supply voltage VDD,And send into the input of comparator C OMP, and reference voltage Vref compares, and works as supply voltageWhen VDD is higher, the output output high level of comparator C OMP, metal-oxide-semiconductor M1 switch closure,The Part I R3-1 of the 3rd resistance R 3, by short circuit, is equivalent to the 3rd resistance R 3 and reduces,K2 increases. In the time that vdd voltage is lower, the output of comparator C OMP is output as 0, MOSPipe M1 switch opens, the resistance of the 3rd resistance R 3 is by Part I R3-1 and Part II R3-2Composition, is equivalent to the 3rd resistance R 3 and increases, and K2 reduces.

Above-described is only the preferred embodiment of the present invention, the invention is not restricted to above enforcementExample. Be appreciated that those skilled in the art are in the prerequisite that does not depart from spirit of the present invention and designThe oher improvements and changes that directly derive or associate down, all should think and be included in guarantor of the present inventionWithin protecting scope.

Claims (2)

1. a RS-485 input common-mode range expanded circuit for adaptive supply voltage, is characterized in that:Comprise biasing circuit, buffer amplifier, input voltage regulation circuit and current potential translation circuit;

Described biasing circuit comprises the first resistance (R1) and the second resistance (R2), described the first resistance (R1)With the second resistance (R2) series connection, this series circuit one end connects supply voltage (VDD), other end ground connection;Described the first resistance (R1) is connected buffer amplifier (Buffer) with the common port of the second resistance (R2)Input;

Described input voltage regulation circuit comprises the 3rd resistance (R3) and the 4th resistance (R4); The described the 3rdResistance (R3) and the series connection of the 4th resistance (R4), this series circuit one end is voltage input end mouth (PINA),The other end connects the output of buffer amplifier (Buffer);

Described current potential translation circuit comprises the first triode (Q1) and the second triode (Q2), described firstTriode (Q1) and the second triode (Q2) are PNN type triode, the first triode (Q1) andThe grounded collector of two triodes (Q2), base stage connection the 3rd resistance (R3) and the 4th resistance (R4)Common port, emitter stage, as the output of circuit, connects the input of comparator circuit in late-class circuit.

2. the RS-485 input common-mode range expanded circuit of adaptive supply voltage as claimed in claim 1,It is characterized in that: described the 3rd resistance (R3) is variable resistor; Also comprise detection-control module(Det-Control), described detection-control module (Det-Control) comprise draw together the 5th resistance (R5),The 6th resistance (R6), comparator (COMP) and N-type metal-oxide-semiconductor (M1); Described the 5th resistance (R5)With the 6th resistance (R6), this series circuit one end connects supply voltage (VDD), other end ground connection; The 5thResistance (R5) is connected the positive input of comparator (COMP) with the common port of the 6th resistance (R6), thanCompared with the negative input input reference voltage (Vref) of device, the output of comparator (COMP) connects MOSThe grid of pipe (M1), the drain electrode of metal-oxide-semiconductor (M1) connects one end of the 3rd resistance (R3), metal-oxide-semiconductor (M1)Source electrode connect the resistance adjustable side of the 3rd resistance (R3).