Datasheet

+
-
V
IN
R
1
R
G
R
F
V
OUT
LMH6611
R
2
V
+
V
+
LMH6611, LMH6612
www.ti.com 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 R
1
, R
1
= R
F
/gain.
11. Calculate R
2
, R
2
= 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.
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. R
1
= 2 kΩ/2 = 1 kΩ
11. R
2
= 2 kΩ/(2.4-2) = 5 kΩ
12. R
G
= 2 kΩ/(2.4 – 1) = 1.43 kΩ
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