Hui Sideng bridge is the classic example of this kind of circumstance, but like biosensor a kind galvanic cell has similar character. Bridge outputs signal to distribute signal for difference, accordingly, appearance amplifier is the first selection that high accuracy measures. Below good case, without laden bridge output is 0, but become only 4 all resistor all identical when, fang Weizhen of this kind of circumstance. If one establishs the bridge that type resistor compose builds in order to divide, if pursue,1 is shown. The poorest condition differs minute of maladjusted VOS to be:
 

 

Among them, VEX is bridge drive voltage, TOL is resistor tolerance (the unit is per cent) .

 

Graph 1 Hui Sideng bridge is maladjusted
 

For example, all be in the tolerance of each component 0.1% and when incentive voltage is 5 V, poor cent is maladjusted and OK be as high as ±5 MV. If need the gain of 400 to realize place to require bridge sensitivity, criterion what amplifier output carries is maladjusted become ±2 V. Hypothesis amplifier by drive of same power source, and its are outputted can course swings to course, criterion only bridge is maladjusted the output amplitude that uses up 80% above possibly. In voltage of trade demand power supply smaller and smaller current falls, this problem can become more bad only.

 

Traditional 3 transport put appearance amplifier framework (if the graph is shown 2 times) have class of a gain that need branch, it is a subtracter ever since, use at moving to divide in all modular voltage. Gain brings to bear on at the first class, accordingly, the multiple of maladjusted enlarge and target signal are identical. Accordingly, move its the only method that eliminate is to consulting (REF) end brings to bear on turn over voltage. The main inadequacy of this kind of method depends on, if the first class of amplifier is already saturated, adjust the voltage on REF can not correct maladjusted. A few kinds of methods that overcome this bit of inadequacy include:

 

* basis is specific circumstance, be opposite with exterior resistor bridge billabong, but to automation production, this is not actual, and after leaving factory, cannot adjusting

* reduces gain of the first class, the voltage that goes up through fine tuning REF will move except maladjusted, add an amplifier circuit to want gain in order to realize place again

* reduces gain of the first class, finish digitlization output with high resolution ADC, move in software except maladjusted

 

Two kinds of option still need to consider the with primitive and maladjusted value error below the poorest condition after, reduce the biggest gain of the first class further thereby. These solutions are not good, because they need space of additional power source, circuit board or cost, will achieve the goal of tall CMRR and low noise. Additional, communicating coupling is not a kind of choice that measure dc or exceeds slow mobile signal.

 

Graph 2 3 carry put appearance amplifier develop attacks structure
 

Indirect electric current feedbacks (ICF) appearance amplifier (like AD8237 and AD8420) the reach that can magnifying is divided maladjusted. Graph develop of 3 indication ICF attacks structural principle graph.

 

Graph amplifier of appearance of feedback of 3 indirect electric current develop attacks structure
 

The transfer function of this appearance amplifier is in formally and classical 3 carry put develop to attack structural transfer function is same, its computation formula is

 

 

Because input the voltage between to be equal to feedback (FB) and reference (when the voltage between REF) terminal, the feedback requirement of amplifier is attainable contented, accordingly, we can rescript this formula for
 

 

This is meant, introduce to be equal to feedback and referenced terminal between maladjusted voltage, although be being put below the circumstance with big maladjusted input, also can adjust output for 0 volt. If the graph is shown 4 times, should adjust can come true through the following method: From a simple voltage source (like low cost DAC) perhaps come from embedded the signal of filter wave PWM of small controller, through resistor RA small report disgorge enters feedback node.

 

Graph 4 belts are maladjusted move the circuit of tall gain bridge that divides a function

 

According to equality (3) , of R1 and R2 it is than setting gain:
 

 

Stylist must decide resistance. Greater resistance can reduce power comsumption and output load; Lesser value can restrict FB input to slant buy electric current and input impedance error. If the shunt-wound combination of R1 and R2 is more than,make an appointment with 30 K? , criterion resistor begins to cause noise. Watch 1 showed a few proposals are worth.

 

The commendation that expresses all sorts of 1 gain resistor (1% resistor)
 

 

To simplify RA is worth search a process, use operation pattern of double power source suppose, have REF of a ground connection terminal mixes foregone bipolar adjust voltage VA. The output voltage below this kind of circumstance can calculate through the following formula:
 

 

Attention, it is to the gain of output from VA instead. The addition of VA can make output voltage drop small, ratio is R2 and RA than. Below this ratio, can be aimed at given the input is maladjusted, make adjust limits to achieve the biggest. Because adjust the amplifier input before limits points to gain, accordingly, although in low resolution the circumstance of the source falls, also can carry out fine tuning. Because RA is general bigger than R1 much, accordingly, we can gain equality (the approximation of 5) :

 

Be worth to find a RA in order to allow the biggest maladjusted adjust limits VIN(MAX) , in give the circumstance that adjusts voltage limits VA(MAX) to fall, make VOUT = 0, beg RA, the result gets

 

 

Among them, what VIN(MAX) anticipates for sensor is the biggest maladjusted. Equality (5) shows at the same time, those who adjust circuit insert initiate to revise from the gain that inputs output. Even so, its effect is general very small also, gain is OK recount is:

 

 

Popularly, recuperate application to bridge of only power source, the voltage of referenced end should be more than signal ground. If bridge output can be in,losing swing, circumstance especially such. If fiducial voltage source by source of a low impedance (if divide block implement with bumper) drive comes voltage VREF, if the graph is shown 5 times, criterion equality (5) turns into:

 

 

If take VOUT and VA relative to the VREF in primitive equality, attainable same result. VA(MAX)–VREF also should replace equality (the VA(MAX) in 7) .

 

 

Have amplifier of bridge of an only power source suppose, if the graph is shown 4 times, among them, come with 3.3 V voltage incentive bridge and drive amplifier. Output of full span bridge is ±15 MV, maladjusted likelihood is in the limits of ±25-mV. Require sensitivity to acquire place, amplifier gain needs to be 100, the input limits of ADC comes for 0 V 3.3 V. Can think as a result of the output of bridge, also can feel negative, accordingly, its output points to intermediate power source or 1.65V. Need to pass only bring to bear on the gain of 100, maladjusted itself can make amplifier output is in –0.85 V to come compulsively namely + inside the limits of 4.15 V, this exceeded power source course.

 

The circuit that this problem can show 5 times through the graph will solve. Bridge amplifier A1 is an ICF appearance amplifier like AD8237. Amplifier A2, take R4 and R5, the 0 n of A1 output is set for intermediate power source. AD5601 8 DAC undertakes adjustment to output, make bridge maladjusted through RA for 0. Next, the output of amplifier by AD7091 small power comsumption 12 ADC are digitlized.

 

Graph 5 what revise in the light of mode of job of only power source is maladjusted move except circuit
 

From the watch 1 can discover, gain is 101 when, do R1 and R2 need to be 1 K? With 100 K? . Circuit includes to be able to come in 0 V swing inside 3.3 V limits, perhaps swing in left and right sides of 1.65V fiducial voltage ±1.65 V. For the value of computational RA, we use equality (6) . Among them, VA(MAX) = 1.65 V and VIN(MAX) = 0.025 V, RA = 65.347k? . It is when resistor tolerance 1% when, is the most adjacent value 64.9 K? . However, this does not have the error that causes for source precision and temperature change to leave any abundant to measure, accordingly, do we choose a common 49.9-k? Low cost resistor. The cost that such doing is to adjusted resolution to reduce, the result is brought about slightly after big adjustment maladjusted.

 

From equality (7) , we are OK rated gain value is cipher out 103. If stylist hopes to get the gain of 100 is worth immediateness a target to be worth, the simplest way is to make the value of R2 is reduced 3% the left and right sides, to 97.6 K? , the result is very small to the influence of the value of RA. Below new requirement, rated gain is 100.6.

 

Can swing as a result of DAC ±1.65 V, accordingly, always maladjusted adjust limits to be able to pass the voltage divider that forms by the shunt-wound combination of RA and R1 and R2 to give, its calculation method is as follows:
 

 

In ±25-mV the biggest bridge is maladjusted inside limits, the adjustment of ±32.1-mV limits can offer the frontal transfer to other localities of 28% to rectify abundant to measure. To 8 DAC, tone synchronization is long for:

 

Adjust resolution to 250-µV, the biggest remains that outputs end is maladjusted for 12.5 MV.

 

The value of R3 and C1 can pass the proposal value in ADC data manual or bibliographical reference 2 will decide. Lead the AD7091 that is 1 MSPS to sampling, are these values 51? With 4.7 NF. Be in with inferior rate sampling when, can use greater resistance or capacitance combination, with decreasing further noise is mixed mix fold effect.

 

Another advantage of this circuit depends on, can be being produced or bridge is finished when installation maladjusted adjust. If arrearage of ambient conditions, sensor or long-term drift are right maladjusted value is influential, criterion but readjust circuit.

 

Suffer its true path inputs an influence to course, AD8237 suits to use the bridge application that exceeds voltage of low power source most. The traditional industry of voltage of taller to the requirement power source applies, AD8420 can yet be regarded as replace parts of an apparatus goodly. Amplifier of this ICF appearance uses 2.7 V to come power supply of 36 V power source, power comsumption is low 60% .

 

Watch 2 it is right two appearance amplifier undertook comparative. Used the smallest with the biggest norms. About more detail and up to date information, refer to product data manual please.

 

Express 2 AD8237 and AD8420 to compare