Datasheet AD8350 (Analog Devices) - 10

AD8350 CAC CAC
The insertion loss and the resultant power gain for multiple shunt resistor values is summarized in Table I. The source
8 7 6 5
resistance and input impedance need careful attention when
RS R R SHUNT L AD8350
using Equation 1. The reactance of the input impedance of the AD8350 and the ac-coupling capacitors need to be considered
– VS
before assuming they have negligible contribution. Figure 10
+
shows the effective power gain for multiple values of RSHUNT for
RS R R SHUNT L
the AD8350-15 and AD8350-20.
1 2 3 4 Table I. Gain Adjustment Using Shunt Resistor, C C AC AC RS = 100 and RIN = 100 Single-Ended 0.1 F ENBL (5V) Power Gain–dB +VS (5V TO 10V) RSHUNT– IL–dB AD8350-15 AD8350-20
Figure 8. Gain Reduction Using Shunt Resistor 50 6.02 8.98 13.98 100 3.52 11.48 16.48 200 1.94 13.06 18.06
RFEXT
300 1.34 13.66 18.66 400 1.02 13.98 18.98
C C AC AC 8 7 6 5 R 20 S RL AD8350 18 – VS 16 AD8350-20 + 14 RS RL 12 1 2 3 4 AD8350-15 10 0.1 C F C AC ENBL AC GAIN – dB 8 (5V) +VS 6 (5V TO 10V) 4 RFEXT 2
Figure 9. Dynamic Gain Reduction
0 0 100 200 300 400 500 600 700 800 RSHUNT –
Figure 8 shows a typical implementation of the shunt divider Figure 10. Gain for Multiple Values of Shunt Resistance concept. The reduced input impedance that results from the for Circuit in Figure 8 parallel combination of the shunt resistor and the input impedance The gain can be adjusted dynamically by employing external of the AD8350 adds attenuation to the input signal effectively feedback resistors as shown in Figure 9. The effective attenua- reducing the gain. For frequencies less than 100 MHz, the input tion is a result of the lowered input impedance as with the shunt impedance of the AD8350 can be modeled as a real 200 Ω resis- resistor method, yet there is no additional noise contribution at tance (differential). Assuming the frequency is low enough to the input of the device. It is necessary to use well-matched resistors ignore the shunt reactance of the input, and high enough such to minimize common-mode offset errors. Quality 1% tolerance that the reactance of moderately sized ac-coupling capacitors resistors should be used along with a symmetric board layout to can be considered negligible, the insertion loss, IL, due to the help guarantee balanced performance. The effective gain for mul- shunt divider can be expressed as: tiple values of external feedback resistors is shown in Figure 11. ⎡ R ⎤ IN ⎢ ⎥ (RIN R ) IL(dB) = 20 × + Log S 10 ⎢ ⎥ R IN RSHUNT ⎢ ⎥ ⎢ (R ⎣ + ⎥ IN RSHUNT RS ) ⎦ where (3) RIN R R SHUNT = × and = 100Ω sin e− IN RSHUNT RIN gl nded e R + IN RSHUNT –10– REV. C