Signal conditioning Equation 6. The error
depends on RL/R0 and k. A smaller load resistor and higher k will decrease the offset error. We can also calculate the temperature drift of the circuit, which comes from amplifiers and resistors. Amplifiers’ offset voltage and bias current change with the work temperature. For most CMOS input amplifiers, the bias current doubles for every increase of 10°C. The drift of resistors changes a lot with different types. For example, carbon composition units’ TC is approximately 1,500 ppm/°C, while metal film and bulk metal resistors’ TC can be 1 ppm/°C.
(6)
Figure 6. Output noise density curve of EHCS based on ADA4870 Table 1. Precision Amplifiers Parameters Choosing a precision amplifier is good for the dc accuracy of the output
current. However, there are many limitations in the precision amplifier selection. The drive capability and ac performance are not good enough. Table 1 lists some common precision amplifiers. We want to build a ±500 mA current source with 1 µs settling time. For a current source we would need high drive capability. For a current source with additional high settling time we need good ac performance. In general, precision amplifiers do not provide that specification combination as the slew rate and bandwidth are not good enough. This requires choosing from a few other amplifiers.
EHCS ImplEmEntatIon
ADA4870 is a high speed, high voltage, and high drive capacity amplifier. It can supply 10 V to 40 V with 1.2 A output current limitation. Its bandwidth is over 52 MHz for a large signal and the slew rate is up to 2,500 V/µs. All these specifications make it the right fit for fast settling and a large current source. Figure 4 shows an EHCS circuit based on the ADA4870 that generates a ±500 mA output current source by 10 V input.
and negative slew rate are +25 A/µs and –25 A/µs. The noise performance is shown in the output noise density curve. It’s about 24 nV/√Hz at 1 kHz. Due to large input offset voltage and bias current, the dc precision is not good in this circuit. Table 2 shows different dc error sources and contribution. The main dc error comes from the Vos and IB of ADA4870. The typical output current offset is about 11.06 mA, which is about 2.21 per cent range error referring to 500 mA full range.
CompoSItE amplIfIEr topology
High drive amplifiers like ADA4870’s dc parameters limit output current accuracy, and high precision amplifiers don’t have enough speed. Here we can combine all these qualities into one circuit with composite amplifier topology. Figure 7 shows the composite amplifier enhanced Howland current source (CAEHCS) that is formed by ADA4870 and ADA4898-2.
Table 2. The DC Error of EHCS Based on ADA4870 Figure 4. EHCS circuit based on ADA4870
In ac specifications, we are more care about settling time, slew rate, bandwidth, and noise. The settling time is about 60 ns and bandwidth is about 18 MHz as shown in Figure 5. The output current slew rate can be calculated by measuring the slope of the rising and falling stage. The positive
ADA4898-2 is chosen to form the composite amplifier for its excellent ac and dc performance. Its –3 dB bandwidth is 63 MHz. The settling time to 0.1 per cent with a 5 V output step is 90 ns and the slew rate is up to 55 V/µs. It has ultralow noise, too. The voltage noise density is 0.9 nV/√Hz and current noise density is 2.4 pA/√Hz. As for dc specifications, it performs well, too. The typical input offset voltage is 20 µV with 1 µV/°C temperature drift. The bias current is 0.1 µA. Table 3 shows the dc error of the CAEHCS. The output current offset is decreased to 0.121 mA, which means the range error is lower than 0.03 per cent. The ac performance of the CAEHCS is
Figure 5. Settling time and frequency response of an EHCS based on ADA4870 Instrumentation Monthly August 2019
shown in Table 4. The settling time and bandwidth are lower than EHCS due to the loop delay of the composite amplifier. CAEHCS output noise is much lower than EHCS output noise due to low current noise of ADA4898-2. As specified in the data sheet, the ADA4870’s invert input current noise density is 47 pA/√Hz. With several kΩ resistors, it will generate much higher noise than the voltage noise (2.1
Continued on page 58... 57
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82
