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AD8350ARMZ15 Datasheet(PDF) 10 Page - Analog Devices |
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AD8350ARMZ15 Datasheet(HTML) 10 Page - Analog Devices |
10 / 14 page REV. AD8350 –10– VS 87 6 5 12 34 AD8350 – ENBL (5V) +VS (5V TO 10V) 0.1 F CAC RS CAC CAC CAC RSHUNT RSHUNT RS RL RL Figure 8. Gain Reduction Using Shunt Resistor VS 87 6 5 12 34 AD8350 – ENBL (5V) +VS (5V TO 10V) 0.1 F RS CAC CAC RS RL RL CAC CAC RFEXT RFEXT Figure 9. Dynamic Gain Reduction Figure 8 shows a typical implementation of the shunt divider concept. The reduced input impedance that results from the parallel combination of the shunt resistor and the input impedance of the AD8350 adds attenuation to the input signal effectively reducing the gain. For frequencies less than 100 MHz, the input impedance of the AD8350 can be modeled as a real 200 Ω resis- tance (differential). Assuming the frequency is low enough to ignore the shunt reactance of the input, and high enough such that the reactance of moderately sized ac-coupling capacitors can be considered negligible, the insertion loss, IL, due to the shunt divider can be expressed as: IL dB Log R RR RR RR R RR RR RR R gl nded IN IN S IN SHUNT IN SHUNT S IN SHUNT IN SHUNT IN SHUNT IN () () () =× + + = × + =− 20 100 10 where and Ω sin e e (3) The insertion loss and the resultant power gain for multiple shunt resistor values is summarized in Table I. The source resistance and input impedance need careful attention when using Equation 1. The reactance of the input impedance of the AD8350 and the ac-coupling capacitors need to be considered before assuming they have negligible contribution. Figure 10 shows the effective power gain for multiple values of RSHUNT for the AD8350-15 and AD8350-20. Table I. Gain Adjustment Using Shunt Resistor, RS = 100 and RIN = 100 Single-Ended Power Gain–dB RSHUNT– IL–dB AD8350-15 AD8350-20 50 6.02 8.98 13.98 100 3.52 11.48 16.48 200 1.94 13.06 18.06 300 1.34 13.66 18.66 400 1.02 13.98 18.98 RSHUNT – 20 0 18 16 14 12 10 8 6 4 2 0 100 200 300 400 500 600 700 800 AD8350-20 AD8350-15 Figure 10. Gain for Multiple Values of Shunt Resistance for Circuit in Figure 8 The gain can be adjusted dynamically by employing external feedback resistors as shown in Figure 9. The effective attenua- tion is a result of the lowered input impedance as with the shunt resistor method, yet there is no additional noise contribution at the input of the device. It is necessary to use well-matched resistors to minimize common-mode offset errors. Quality 1% tolerance resistors should be used along with a symmetric board layout to help guarantee balanced performance. The effective gain for mul- tiple values of external feedback resistors is shown in Figure 11. B |
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