LMH6551 Datasheet, PDF (18/32 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6551
SNOSAK7D – FEBRUARY 2005 – REVISED JANUARY 2015
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Typical Applications (continued)
9.2.3 Single Supply Operation
The input stage of the LMH6551 has a built in offset of 0.7 V towards the lower supply to accommodate single
supply operation with single-ended inputs. As shown in Figure 29, the input common-mode voltage is less than
the output common voltage. It is set by current flowing through the feedback network from the device output. The
input common-mode range of 0.4 V to 3.2 V places constraints on gain settings. Possible solutions to this
limitation include AC coupling the input signal, using split power supplies and limiting stage gain. AC coupling
with single supply is shown in Figure 30.
In Figure 29 closed loop gain = V_O / V_I ≊ R_F / R_G, where V_I =V_S / 2, as long as R_M << R_G. Note that in single-
ended to differential operation V_I is measured single-ended while V_O is measured differentially. This means that
gain is really 1/2 or 6 dB less when measured on either of the output pins separately. Additionally, note that the
input signal at R_T (labeled as V_I) is 1/2 of V_S when R_T is chosen to match R_S to R_IN.
V_ICM = Input common-mode voltage = (V_I1+V_I2) / 2.
R_F
R_S
= ^VI
R_G
R_T ^VCM
R_G
R_M
V_I1
+
-
V_I2
R_F
R_O
V_O1
C_L R_L V_O
V_O2
R_O
V_O1 + V_O2
*V_CM =
2
*BY DESIGN
V_ICM = V_OCM
V_I1 + V_I2
V_ICM =
2
Figure 30. AC-Coupled for Single Supply Operation
18
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