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CLC425AJE Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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CLC425AJE Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 9 page As seen in Figure 5, eni is dominated by the intrinsic voltage noise (en) of the amplifier for equivalent source resistances below 33.5 Ω. Between 33.5Ω and 6.43kΩ, eni is dominated by the thermal noise (e 4TR t seq = k ) of the external resistors. Above 6.43k Ω, e ni is dominated by the amplifier's current noise ( 2i R n seq ). The point at which the CLC425's voltage noise and current noise contribute equally occurs for Rs eq=464Ω (i.e. e2i nn / ). As an example, configured with a gain of +20V/V giving a -3dB of 90MHz and driven from an Rs eq=25Ω, the CLC425 produces a total equivalent input noise voltage (e 1.57 90MHz ni ∗∗ ) of 16.5 µV rms. Figure 5: Voltage Noise Density vs. Source Resistance If bias current cancellation is not a requirement, then Rf||Rg does not need to equal Rs eq. In this case, according to Equation 1, Rf||Rg should be as low as possible in order to minimize noise. Results similar to Equation 1 are obtained for the inverting configuration of Figure 2 if Rs eq is replaced by Rb and Rg is replaced by Rg+Rs. With these substitutions, Equation 1 will yield an eni refered to the non-inverting input. Refering eni to the inverting input is easily accomplished by multiplying eni by the ratio of non-inverting to inverting gains. Noise Figure Noise Figure (NF) is a measure of the noise degradation caused by an amplifier. The Noise Figure formula is shown in Equation 3. The addition of a terminating resistor RT, reduces the external thermal noise but increases the resulting NF. The NF is increased because RT reduces the input signal amplitude thus reducing the input SNR. Rseq = Rs for Unterminated Systems Rseq = Rs II RT for Terminated Systems Equation 3: Noise Figure Equation The noise figure is related to the equivalent source resistance (Rs eq) and the parallel combination of Rf and Rg. To minimize noise figure, the following steps are recommended: • Minimize R f||Rg • Choose the optimum R s (ROPT) ROPT is the point at which the NF curve reaches a minimum and is approximated by: ROPT ≅ (e n/in) Figure 6 is a plot of NF vs Rs with Rf||Rg = 9.09 (Av = +10). The NF curves for both Unterminated and Terminated systems are shown. The Terminated curve assumes Rs = RT. The table indicates the NF for various source resistances including Rs = ROPT. Figure 6: Noise Figure vs Source Resistance Supply Current Adjustment The CLC425's supply current can be externally adjusted downward from its nominal value by adding an optional resistor (Rp) between pin 8 and the negative supply as shown in Figure 7. Several of the plots found within the plot pages demonstrate the CLC425’s behavior at different supply currents. The plot labeled “Icc vs. Rp” provides the means for selecting Rp and shows the result of standard IC process variation which is bounded by the 25°C curve. Figure 7: External Supply Current Adjustment Non-Inverting Gains Less Than 10V/V Using the CLC425 at lower non-inverting gains requires external compensation such as the shunt compensation as shown in Figure 8. The quiescent supply current must also be reduced to 5mA with Rp for stability. The com- pensation capacitors are chosen to reduce frequency response peaking to less than 1dB. The plot in the "Typical Performance" section labeled “Differential Gain and Phase” shows the video performance of the CLC425 with this compensation circuitry. 3 2 4 7 6 8 +Vcc -Vcc Vout Rp CLC425 NF 10LOG S/ N S/ N 10LOG e e ii oo ni 2 t 2 = = NF 10LOG e i R R | | R 4kTR 4kT R | | R 4kTR n 2 n 2 seq f g 2 seq f g seq = ++ () ++ () http://www.national.com 6 |
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