Although bigger R2 can reduce circuit speed and precision, but in feedback road by in insert a bumper, form to increase model Howland current source can solve this one problem, if pursue,2 are shown. All electricity that carry R0 flow into RL. Export electricity by formula 2 computation.

 

Graph 2. Increase model circuit of Howland current source.

 

 

If R1/R2 = R3/R4 = K, criterion this formula turns into formulary 3. Output electric current and load have nothing to do, get input voltage control only. This is an ideal VCCS.

 

 

Function analysis

Formulary 3 be based on a good system. Graph the 3 dc errors that showed EHCS analyse a model. VOS and IB+/IB– are advocate the input of amplifier is maladjusted voltage and slant buy electric current. The input that VOSbuf and IBbuf are bumper is maladjusted voltage and slant buy electric current. Total output error can adopt formula 4 computation.

 

Graph 3. Maladjusted voltage is calculated.

 

 

Of oversight gain resistor do not match, consider R1/R2 = R3/R4= K, r1//R2= R3//R4. Output maladjusted electric current to depend on of amplifier maladjusted and slant buy electric current, if formula is shown 5 times.

 

 

Consider R1/R2 and R3/R4 do not match, RL will affect output maladjusted electric current. The poorest opposite error is shown 6 times like formula. This error depends on RL/R0 and K. Reduce laden resistance and raise K to will reduce maladjusted error.

 

 

We still can be calculated the temperature drift of circuit, it comes from amplifier and resistor. The maladjusted voltage of amplifier and slant to buy electric current follows working temperature and change. To amplifier of most CMOS input, temperature every are elevatory 10 ℃ , slant buy electric current increases one times. The drift change of different type resistor is very big. For example, the TC of carbolic film resistor is 1500 Ppm/ ℃ about, and the TC of metallic film and body metal resistor may be 1 Ppm/ ℃ .

 

Choose accurate amplifier to be helpful for outputting dc precision of electric current. However, the choice of accurate amplifier also is put in a lot of limitation. Its drive ability and communication performance are not quite good. Express 1 a few listedder common accurate amplifier.

 

Express 1. Accurate amplifier parameter

 

We hope compose establishs the current source of a ±500 MA, set-up time is 1 μs. To current source, we need high drive capability. The current source of fast to be being had even set-up time, we need excellent communication performance. Generally speaking, accurate amplifier cannot offer these two normative combination, it is not quite good to because its are pressed,place rate and bandwidth. This need undertakes choosing from inside the amplifier of other type.

 

EHCS comes true

ADA4870 is the amplifier of ability of a high speed, tall voltage, high drive. It can offer 10 V to come 40 V voltage, limitation of output electric current is 1.2 A. The bandwidth below big signal exceeds 52 MHz and pressure place rate be as high as 2500 V/μs. All these norms make it suits to be built quickly very much with big current source. Graph 4 showed the EHCS circuit that is based on ADA4870, it can generate a ±500 MA to output current source through 10 V input.

 

Graph 4. Be based on the EHCS circuit of ADA4870.

 

In communicating norms, we care set-up time more, pressure place rate, bandwidth and noise. If the graph is shown 5 times, set-up time is 60 Ns about, bandwidth is 18 MHz about. Output voltaic pressing to place rate is OK through measuring ascendant phase is mixed drop the slope of level will calculate. negative pressure is placed rate it is respectively + 25 A/μs and –25 A/μs. Curve of output noise density showed noise performance, it is 24 NV/√Hz about when 1 KHz.

 

Graph 5. The EHCS set-up time that is based on ADA4870 and frequency are answered.

 

Graph 6. The EHCS that is based on ADA4870 outputs noise density curve.

 

As a result of the input maladjusted voltage is mixed slant buy electric current is bigger, the dc precision of this circuit is not tall. Watch 2 showed different dc error source and contribution. Main dc error comes from the Vos of ADA4870 and IB. Attune of prediction of a person’s luck in a given year of typical output report is 11.06 MA about, this is equivalent to the 2.21% error limits of the left and right sides when 500 MA whole journey.

 

Express 2. Be based on the EHCS dc error of ADA4870

 

Compound amplifier technology

The dc parameter of such high drive amplifier restricted ADA4870 to output voltaic precision, and the speed of high accuracy amplifier insufficient. For this, we can use compound amplifier technology to be in individual circuit compositive and all these character. The graph is shown 7 times for compound amplifier increases model Howland current source (CAEHCS) , it is comprised by ADA4870 and ADA4898-2.
 

 

Graph 7. Circuit of compound amplifier EHCS.

 

Because it has outstanding communication and dc property,choosing ADA4898-2 to form compound amplifier is. Its – 3 DB bandwidth is 63 MHz. It is in output rank jumps the 0.1% set-up time when be 5 V to be 90ns, pressure place rate can amount to 55 V/µs. It still is had exceed low noise. Voltage noise density is 0.9 NV/√Hz, voltaic noise density is 2.4 PA/√Hz. As to dc norms parameter, its property performance is very good also. The model inputs maladjusted voltage to be 20 µV, temperature drift is 1 µV/°C. Slant buy electric current is 0.1 µA. Express the 3 dc errors that showed CAEHCS. Attune of output report prediction of a person’s luck in a given year is reduced to 0.121 MA, this means error limits to be in 0.03% the following.

 

Express 3. Be based on the CAEHCS dc error of ADA4898

 

If the communication function of CAEHCS expresses 4 to show. As a result of the annulus road defer of compound amplifier, its set-up time and bandwidth all under EHCS. Because the voltaic noise of ADA4898-2 is low, the output noise of CAEHCS is accordingly far the output noise under EHCS. What in be like data manual, indicate, the density of noise of retrorse input electric current of ADA4870 is 47 PA/√Hz. Through using the resistor that block of class of a few KΩ is worth, it compares generation voltage noise (the noise with a lot of louder 2.1 NV/√Hz) . However, the density of noise of input electric current in CAEHCS is 2.4pA/√Hz. The output noise that it produces wants a lot of lower.

 

Express the communication standards of 4.CAEHCS

 

Above all, CAEHCS raised the dc precision of VCCS greatly, have coequal drive ability and communication property. In addition, the compound amplifier product that can offer an alternative is very much, in order to satisfy different requirement. Express the 5 function that amplifier differs in showing CAEHCS circuit. The communication function of LT6275 is best. Its set-up time can amount to 100 Ns less than, pressure place rate be as high as 15 A/µs. The 0 drift amplifier such as ADA4522-2 suits to output electric prediction of a person’s luck in a given year to move an error to be the high accuracy of 0.002 MA to apply about very much.

 

In expressing 5.CAEHCS advocate the choice of amplifier

 

Test result

If the function of the EHCS that is based on ADA4898 and CATHCS is expressed 6 with the graph 8 are shown.

 

Express the comparison of 6.EHCS and CAEHCS

 

Graph 8.ADA4898-2 (CH1- input, CH2- is outputted) set-up time.

 

CAEHCS circuit has a lot of better than EHCS circuit dc norms. Its output attune of electric prediction of a person’s luck in a given year to be 0.2 MA, and the attune of output report prediction of a person’s luck in a given year of EHCS circuit is 10.9 MA. CAEHCS circuit also has very good communication standards. Both set-up time all is 100 Ns. The bandwidth of EHCS circuit is 18 MHz, and the bandwidth of CAEHCS circuit is 8 MHz.

 

If the CAEHCS function that is based on ADA4522-2 and LT6275 expresses 7 to show.

 

In expressing 7.CAEHCS, differ advocate the test result of amplifier

 

The output of ADA4522-2 version is maladjusted the error is smaller, low to 0.04 MA. The set-up time of LT6275 is 60 Ns about, output voltaic pressing to place rate be as high as 16.6A/µs (if pursue,show 9 times) .

 

Graph 9.LT6275 (CH1- input, CH2- is outputted) set-up time.

 

Medicinal powder hot consideration

The output electric current of VCCS can achieve hundreds of milliampere. Integral power comsumption can amount to a few tile. If output efficiency is not tall, the temperature of parts of an apparatus will rise quickly. The thermal resistance when ADA4870 does not use radiator (θJA) is 15.95 ℃ / W. Temperature rise can use formula 7 computation.

 

 

The extraction of R0 is worth the power comsumption that will affect ADA4870. Watch 8 showed the temperature rise that chooses different R0 value below voltage of ±20 V power source. When choosing bigger R0, temperature rise can be reduced greatly. Accordingly, the proposal uses bigger R0 in order to reduce temperature rise.

 

The relation of the power comsumption that expresses 8.ADA4870 and temperature rise and R0 (Io = 500 MA)

 

Conclusion

CAEHCS circuit combines high drive amplifier and high accuracy amplifier, outstanding communication and dc function and big output capacity can be provided in VCCS application. The proposal is united in wedlock in ADA4870 of this circuit lieutenant general and ADA4898, LT6275 and ADA4522 use.

(origin: Yadenuo semiconductor)