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OPA843ID Datasheet(PDF) 11 Page - Texas Instruments |
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OPA843ID Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 29 page  OPA843 11 SBOS268C www.ti.com the 3rd-harmonic much lower. 2-tone 3rd-order intermodulation terms will be much lower than most other solutions using the circuit shown on the front page. The differential typical charac- teristic curves also show that a 4VPP output will have > 80dBc SFDR through 20MHz using this differential approach. WIDE DYNAMIC RANGE “IF” AMPLIFIER The OPA843 offers an attractive alternative to standard fixed- gain IF amplifier stages. Narrowband systems will benefit from the exceptionally high 2-tone 3rd-order intermodulation inter- cept, as shown in the Typical Characteristics. Op amps with high open-loop gain, like the OPA843, provide an intercept that decreases with frequency along with the loop gain. The OPA843’s 3rd-order intercept shows a decreasing intercept with frequency. The OPA843’s intercept is > 30dBm up to 50MHz but improves to > 50dBm as the operating frequency is reduced below 10MHz. Broadband systems will also benefit from the very low even-order harmonics and intermodulation components produced by the OPA843. Compared to standard fixed-gain IF amplifiers, the OPA843 operating at IF’s below 50MHz provides much higher intercepts for its quiescent power dissipation (200mW), superior gain accuracy, higher reverse isolation, and lower I/O return loss. The noise figure for the OPA843 will be higher than alternative fixed-gain stages. If the application comes late in the amplifier chain with significant gain in prior stages, this higher noise figure may be acceptable. Figure 3 shows an example of a noninverting configuration for the OPA843 used as an IF amplifier. 1dB through 50MHz. For narrowband IF’s in the 44MHz region, this configuration of the OPA843 will show a 3rd-order intercept of 33dBm while dissipating only 200mW (23dBm) power from ±5V supplies. PHOTODIODE TRANSIMPEDANCE AMPLIFIER High Gain Bandwidth Product (GBP) and low input voltage and current noise make the OPA843 an ideal wideband transimpedance amplifier for low to moderate gains. Note that unity-gain stability is not required for transimpedance applications. Figure 4 shows an example photodiode ampli- fier circuit. The key parameters of this design are the esti- mated diode capacitance (CD) at the applied DC reverse bias voltage (–VB), the desired transimpedance gain (RF), and the GBP for the OPA843 (800MHz). With these three variables set (and adding the OPA843’s parasitic input capacitance to the value of CD to get CS), the feedback capacitor value (CF) is selected to provide stability for the transimpedance fre- quency response. The input signal and the gain resistor are AC-coupled through the 0.01 µF blocking capacitors. This holds the DC input and output operating point at ground independent of source im- pedance and gain setting. The RG value in Figure 3 (144Ω), sets the gain to the matched load at 12dB. Using standard 1% tolerance resistors for RF and RG will hold the gain to a ±0.2dB tolerance. This example will give a –3dB bandwidth of ap- proximately 100MHz while maintaining gain flatness within To achieve a maximally flat 2nd-order Butterworth frequency response, the feedback pole should be set to: 1 24 ππ RC GBP RC FF F S CCC S DI = =+ (1) Adding the OPA843’s common-mode and differential mode input capacitances CI = (1.0 + 1.2)pF to the 20pF diode source capacitance of Figure 4, and targeting a 10k Ω tran- simpedance gain using the 800MHz GBP for the OPA843, the required feedback pole frequency is 16.9MHz. This will require a total feedback capacitance of 0.94pF. Typical surface-mount resistors have a parasitic capacitance of 0.2pF, leaving the required 0.75pF value shown in Figure 4 to get the required feedback pole. This will set the –3dB bandwidth according to: F GBP RC Hz dB F S − ≅ 3 2 π (2) The example of Figure 4 will give approximately 24MHz –3dB bandwidth using the 0.75pF feedback compensation. OPA843 +5V +5V R S 50 Ω V O P I P 0 0.01 µF R G 144 Ω 1k Ω 52.3 Ω R F 1k Ω 50 Ω Source 50 Ω Load 0.01 µF Power-supply decoupling not shown. Gain = P I P O = 20log 1 2 1 + R F R G dB = 12dB with valuesshown FIGURE 3. High Dynamic Range IF Amplifier. R F 10k Ω Power-supply decoupling not shown. λ OPA843 +5V –5V –V B C F 0.75pF I D V O = IDRF C D 20pF 0.01 µF10kΩ FIGURE 3. High Dynamic Range IF Amplifier. |
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