LMH6611_14 Datasheet, PDF (29/42 Pages) National Semiconductor (TI) – LMH6611/LMH6612 Single Supply 345 MHz Rail-to-Rail Output Amplifiers

LMH6611, LMH6612
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SNOSB00K – NOVEMBER 2007 – REVISED OCTOBER 2013
DC LEVEL SHIFTING
Often a signal must be both amplified and level shifted while using a single supply for the op amp. The circuit in
Figure 74 can do both of these tasks. The procedure for specifying the resistor values is as follows.
1. Determine the input voltage.
2. Calculate the input voltage midpoint, V_INMID = V_INMIN + (V_INMAX – V_INMIN)/2.
3. Determine the output voltage needed.
4. Calculate the output voltage midpoint, V_OUTMID = V_OUTMIN + (V_OUTMAX – V_OUTMIN)/2.
5. Calculate the gain needed, gain = (V_OUTMAX – V_OUTMIN)/(V_INMAX – V_INMIN)
6. Calculate the amount the voltage needs to be shifted from input to output, ΔV_OUT = V_OUTMID – gain x V_INMID.
7. Set the supply voltage to be used.
8. Calculate the noise gain, noise gain = gain + ΔV_OUT/V_S.
9. Set R_F.
10. Calculate R₁, R₁ = R_F/gain.
11. Calculate R₂, R₂ = R_F/(noise gain-gain).
12. Calculate R_G, R_G= R_F/(noise gain – 1).
Check that both the V_IN and V_OUT are within the voltage ranges of the LMH6611.
V⁺
V⁺
R₁
V_IN
R₂
+
LMH6611
-
V_OUT
R_G
R_F
Figure 74. DC Level Shifting
The following example is for a V_IN of 0V to 1V with a V_OUT of 2V to 4V.
1. V_IN = 0V to 1V
2. V_INMID = 0V + (1V – 0V)/2 = 0.5V
3. V_OUT = 2V to 4V
4. V_OUTMID = 2V + (4V – 2V)/2 = 3V
5. Gain = (4V – 2V)/(1V – 0V) = 2
6. ΔV_OUT = 3V – 2 x 0.5V = 2
7. For the example the supply voltage will be +5V.
8. Noise gain = 2 + 2/5V = 2.4
9. R_F = 2 kΩ
10. R₁ = 2 kΩ/2 = 1 kΩ
11. R₂ = 2 kΩ/(2.4-2) = 5 kΩ
12. R_G = 2 kΩ/(2.4 – 1) = 1.43 kΩ
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