LMH6618 Datasheet, PDF (21/26 Pages) National Semiconductor (TI) – 130 MHz, 1.25 mA Rail-to-Rail Input and Output Operational Amplifier with Shutdown

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 6
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 LMH6618.
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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FIGURE 6. DC Level Shifting
4^th ORDER MULTIPLE FEEDBACK LOW-PASS FILTER
Figure 7 shows the LMH6618 used as the amplifier in a mul-
tiple feedback low-pass filter. This filter is set up to have a
gain of +1 and a −3 dB point of 1 MHz. Values can be deter-
mined by using the WEBENCH^® Active Filter Designer found
at amplifiers.national.com.
FIGURE 7. 4^th Order Multiple Feedback Low-Pass Filter
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