Signal conditioning
Figure 8. The settling time of the ADA4898-2 (CH1-Input, CH2-Output) The CAEHCS circuit has much
Figure 7. EHCS circuit with composite amplifier
nV/√Hz). While the input current noise density of the CAEHCS is 2.4 pA/√Hz. It will generate much lower output noise. Above all, CAEHCS has greatly improved the dc accuracy of the VCCS with comparable drive capacity and ac performance. Besides, there are many selections of the composite amplifiers for different requirements. Table 5 shows the performance of different amplifiers in the CAEHCS circuit. LT6275 is the best in ac performance. Its settling time can be within 100 ns, and the slew rate is up to 15 A/µs. Zero-drift amplifiers like ADA4522-2 are suitable for high precision applications that have about 0.002 mA output current offset error.
TesT ResulTs
The performance of the EHCS and CAEHCS based on ADA4898 are shown in Table 6 and Figure 8.
better dc specifications than an EHCS circuit. Its output current offset is 0.2 mA, while the EHCS circuit’s output current offset is 10.9 mA. The CAEHCS circuit has good ac specifications, too. The settling time both are about 100 ns. The bandwidth of the EHCS circuit is 18 MHz, and the CAEHCS circuit is 8 MHz. The performance of the CAEHCS based on the ADA4522-2 and LT6275 is shown in Table 7. The output offset error of ADA4522-2 version is lower to 0.04 mA. The settling time of LT6275 version is about 60 ns and the output current slew rate is up to 16.6 A/µs, which is shown in Figure 9.
Figure 9. The settling time of the LT6275 (CH1-Input, CH2-Output
Table 7. Test Results of the Different Main Amplifier in CAEHCS TheRmal ConsideRaTion
Table 3. The DC Error of CAEHCS Based on ADA4898
The output current of the VCCS can be several hundred milliamperes. The whole power dissipation can be several watts. If output efficiency is bad, the temperature of the par t will rise rapidly. The thermal resistance (θJA) of ADA4870 without a head sink can be 15.95°C/W. The temperature rising can be calculated by using Equation 7.
(7)
Table 8 shows the temperature rise with different R0 selected at a ±20 V supply. The temperature rising will decrease greatly when larger R0 is used. Therefore, larger R0 is recommended to decrease the temperature rise.
The value of R0 will influence the power dissipation of ADA4870. Table 4. The AC Specification of the CAEHCS
Table 5. Selection of Main Amplifier in CAEHCS
Table 8. ADA4870’s Power Dissipation and Temperature Rise vs. R0 (Io = 500 mA)
ConClusion
The CAEHCS circuit that combines a high drive amplifier and a high precision amplifier can provide excellent ac and dc performance with large output capacity in VCCS applications. ADA4870 combined with ADA4898, LT6275, and ADA4522 are recommended for use in this circuit.
Table 6. Comparison of EHCS vs. CAEHCS 58
Analog Devices
www.analog.com August 2019 Instrumentation Monthly
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