Signal conditioning
Table 3. Noise Reduction Using Different Front-End Amplifiers with Effective Gain = 100 at f = 1 kHz Figure 13. Noise sources of composite amplifier.
Table 4. THD+n Comparison Using Different Front-End Amplifiers with Effective Gain = 10 at f = 1 kHz and ILOAD = 200 mA
Table 5. AD8599 + AD8397 Composite Amplifier Specifications
Table 6. Amplifiers with High Output Current Drive
Figure 14. Noise performance vs. stage 1 bandwidth.
shows the frequency response for various AMP1 bandwidths as well as that of AMP2 for a single fixed bandwidth. Recall from the section on gain splitting that a composite gain of 100 (40 dB) and AMP2 gain of 5 (14 dB) will force an effective AMP1 gain of 20 (26 dB) as can be seen here. The bottom plot shows the wideband output
noise density for each case. At low frequencies, the output noise density is dominated by AMP1 (1 nV/√Hz times the composite gain of 100 equals 100 nV/√Hz). This will continue for as long as AMP1 has enough bandwidth to compensate for AMP2. For the cases where AMP1 has less bandwidth
than AMP2, the noise density will begin to be dominated by AMP2 as AMP1 bandwidth begins to roll off. This can be seen in two of the traces of Figure 14 as the noise climbs to 200 nV/√Hz (40 nV/√Hz times the AMP2 gain of 5). Lastly, in the case where AMP1 has much greater bandwidth than AMP2, resulting in a peaking in the frequency response, the composite amplifier will exhibit a noise peak at the same frequency, also shown in
Table 7. Precision Front-End Amplifiers
Figure 14. Since the frequency response peaking results in excessive gain, the amplitude of the noise peak will also be higher. Table 3 and Table 4 shows the effective noise
reduction and the THD+n improvement when using various precision amplifiers as the first stage in a composite amplifier with the AD8397. In this example, the goal for a DAC output
buffer application is to provide an output of 10 V p-p into a low impedance probe with a current of 500 mA p-p, low noise and distortion, excellent dc precision, and as high of a bandwidth as possible. The output of a 4 mA to 20 mA current-out DAC is to be converted to a voltage by the TIA, then to the input of the composite amplifier for more amplification. With AD8397s on the output, the output requirements are attainable. AD8397 is a rail-to-rail, high output current amplifier capable
of delivering the needed output current. AMP1 could be any precision amplifier that has
the desired dc precision needed for the configuration requirement. In this application, various front-end precision amplifiers could be used with AD8397 (and other high output current amplifiers) to attain both the excellent dc requirements and the high output capability drive needed for the application. This configuration is not limited to AD8397 and
AD8599, but is possible with other combinations of amplifiers to cater this output drive specification that requires excellent dc precision. The amplifiers in Table 6 and Table 7 are also suited for this application.
ConClusion
With the composite amplifier, the marriage of two amplifiers realises the best specifications that each one offers while compensating for their limitations. Amplifiers with high output drive capability combined with precision front-end amplifiers could provide solutions to applications with challenging requirements. When designing, always consider stability, noise peaking, bandwidth, and slew rate for optimum performance. There are plenty of possible options to cater a wide range of applications. With the proper implementation and combination, striking the right balance for the application is highly achievable.
Figure 15. Application circuit for DAC output driver. 56
Figure 16. VOUT and IOUT for AD8599 and AD8397 composite amplifier.
Analog Devices
www.analog.com January 2021 Instrumentation Monthly
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
