Voltage-controlled current sources (VCCS) are widely used in many fields such as medical devices and industrial automation. The DC accuracy, AC performance, and drive capability of the VCCS are critical in these applications. This article analyzes the limitations of the Enhanced Howland Current Source (EHCS) circuit and illustrates how it can be improved with a composite amplifier topology to achieve a highly accurate, fast-settling 500 mA current source.
Figure 1 shows a conventional Howland current source (HCS) circuit, while Equation 1 shows how the output current is calculated. If R2 is large enough, the output current will remain constant.
Figure 1Howland current source circuit.
Although the larger R2 reduces the speed and accuracy of the circuit, inserting a buffer into the feedback routing to form an enhanced howland current source can solve the problem, as shown in Figure 2. All current through R0 flows into Rl. The output current is calculated by Equation 2.
Figure 2Enhanced Howland current source circuitry.
If r1 r2 = r3 r4 = k, then the formula becomes Formula 3. The output current is load independent and is controlled only by the input voltage. This is an ideal VCCS.
Equation 3 is based on an ideal system. Figure 3 shows the DC error analysis model for EHCS. VOS and IB+ IB are the input offset voltage and bias current of the main amplifier. Vosbuf and IBBUF are the input offset voltage and bias current of the buffer. The total output error can be calculated by Equation 4.
Figure 3Offset voltage calculation.
Ignore the mismatch of the gain resistors and consider r1 r2 = r3 r4 = k, r1 r2 = r3 r4. The output offset current depends on the offset and bias currents of the amplifier, as shown in Equation 5.
Considering the mismatch between R1 R2 and R3 R4, RL will affect the output offset current. The worst-case relative error is shown in Equation 6. This error depends on rl r0 and k. Decreasing the load resistance and increasing k will reduce offset errors.
We can also calculate the temperature drift of the circuit, which comes from the amplifier and the resistor. The offset voltage and bias current of the amplifier vary with the operating temperature. For most CMOS input amplifiers, the bias current doubles for every 10 temperature increases. The drift of different types of resistors varies greatly. For example, a TC for a carbon film resistor is about 1500 ppm, while a TC for a metal film and bulk metal resistor may be 1 ppm.
Choosing a precision amplifier is beneficial for the DC accuracy of the output current. However, there are also a number of limitations to the choice of precision amplifiers. Its drive capacity and AC performance are not good enough. Table 1 lists some common precision amplifiers.
Table 1Precision amplifier parameters.
We want to build a 500 mA current source with a settling time of 1 s. For current sources, we need a high drive capacity. For a current source that also has a fast settling time, we need excellent AC performance. In general, precision amplifiers do not offer a combination of these two specifications because their slew rate and bandwidth are not good enough. This requires a choice from other types of amplifiers.
The ADA4870 is a high-speed, high-voltage, high-drive amplifier. It is available from 10 V to 40 V with an output current limit of 12 a。Bandwidths of more than 52 MHz and slew rates of up to 2500 V s at large signals. All of these specifications make it ideal for fast set-up and high current sources. Figure 4 shows an EHCS circuit based on the ADA4870 that generates a 500 mA output current source from a 10 V input.
Figure 4EHCS circuit based on ADA4870. In AC specifications, we are more concerned with settling time, slew rate, bandwidth, and noise. As shown in Figure 5, the settling time is about 60 ns and the bandwidth is about 18 MHz. The output current slew rate can be calculated by measuring the slope of the rising and falling phases. The positive and negative slew rates are +25 a s and 25 a s, respectively. The output noise density curve shows the noise performance, which is approximately 24 nV Hz at 1 kHz.
Figure 5EHCS based on ADA4870 to establish time and frequency response.
Figure 6EHCS output noise density curve based on ADA4870.
Due to the large input offset voltage and bias current, the DC accuracy of this circuit is not high. Table 2 shows the different DC error sources and their contributions. The main DC error comes from the VOS and IB of the ADA4870. The typical output current offset is about 1106 mA, which is equivalent to 500 mA full 2Margin of error of about 21%.
Table 2Based on the ADA4870 EHCS DC error.
The DC parameters of a high drive amplifier such as the ADA4870 limit the accuracy of the output current, and the speed of the high-precision amplifier is insufficient. To do this, we can integrate all of these characteristics in a single circuit using compound amplifier technology. Figure 7 shows a composite amplifier enhanced Howland current source (CAEHCS) consisting of the ADA4870 and ADA4898-2.
Figure 7Composite amplifier EHCS circuit.
The ADA4898-2 was chosen for its excellent AC and DC performance. Its -3 dB bandwidth is 63 MHz. It is 0. at an output step of 5 VThe 1% settling time is 90ns and the slew rate can reach 55 V s. It also has ultra-low noise. The voltage noise density is 09 nV Hz with a current noise density of 24 pa/√hz。As for the DC specs, it also performs very well. The typical input offset voltage is 20 V and the temperature drift is 1 V °C. The bias current is 01 µa。Table 3 shows the DC error of the CAEHCS. The output runaway is reduced to 0121 mA, which means a margin of error of 003% or less.
Table 3CAEHCS DC error based on ADA4898.
The AC performance of CAEHCS is shown in Table 4. Due to the loop delay of the composite amplifier, its settling time and bandwidth are lower than those of EHCS. Due to the low current noise of the ADA4898-2, the output noise of the CAEHCS is much lower than that of the EHCS. As indicated in the data sheet, the ADA4870 has a reverse input current noise density of 47 Pa Hz. By using a resistor of several k-class resistances, it will produce specific voltage noise (21 NV Hz). However, the input current noise density in CAEHCS is 24pa/√hz。It produces much lower output noise.
Table 4AC specifications for CAEHCS.
First of all, CAEHCS greatly improves the DC accuracy of VCCs, and has the same driving ability and AC performance. In addition, there are many compound amplifier products to choose from to meet different needs. Table 5 shows the performance of the different amplifiers in the CAEHCS circuit. The LT6275 has the best AC performance. It has an settling time of less than 100 ns and a slew rate of up to 15 A s. Zero-drift amplifiers such as the ADA4522-2 are ideal for output offset errors of about 0High-precision applications for 002 mA.
Table 5Selection of the main amplifier in the CAEHCS.
The performance of the ADA4898-based EHCS and CATHCS is shown in Table 6 and Figure 8.
Table 6Comparison of EHCS with CAEHCS.
Figure 8The settling time of ADA4898-2 (CH1-input, CH2-output).
CAEHCS circuits have much better DC specifications than EHCS circuits. Its output current offset is 02 mA, while the output current offset of the EHCS circuit is 109 ma。The CAEHCS circuits also have very good AC specifications. Both have an settling time of 100 ns. The bandwidth of the EHCS circuit is 18 MHz, while the bandwidth of the CAEHCS circuit is 8 MHz.
Table 7 shows the CAEHCs performance based on ADA4522-2 and LT6275.
Table 7Test results for different main amplifiers in CAEHCS.
The ADA4522-2 version has a lower output offset error as low as 004 ma。The LT6275 has an settling time of approximately 60 ns and an output current slew rate of up to 166a s (as shown in Figure 9).
Figure 9The settling time of LT6275 (CH1-input, CH2-output).
The output current of the VCCS can reach several hundred milliamps. The overall power consumption can reach several watts. If the output efficiency is not high, the temperature of the device will rise rapidly. The ADA4870 has a thermal resistance (ja) of 15 when not using a heat sink95℃/w。The temperature rise can be calculated using Equation 7.
The value of R0 affects the power consumption of the ADA4870. Table 8 shows the temperature rise at a 20 V supply voltage with different R0 values. When a larger r0 is used, the temperature rise is greatly reduced. Therefore, a larger r0 is recommended to reduce the temperature rise.
Table 8The power consumption and temperature rise of the ADA4870 as a function of R0 (IO = 500 mA).
Conclusion
The CAEHCS circuit combines a high-drive amplifier with a high-precision amplifier to provide excellent AC and DC performance as well as large output capacity in VCCS applications. It is recommended to use the ADA4870 in combination with the ADA4898, LT6275, and ADA4522 in this circuit.
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