LMH6503 Datasheet, PDF (14/20 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear Variable Gain Amplifier

Typical Performance Charateristics Unless otherwise specified: V_S = ±5V, 25˚C, V_G = V_{G_MAX}, V_CM
= 0V, R_F = 1kΩ, R_G = 174Ω, both inputs terminated in 50Ω, R_L = 100Ω, Typical values, results referred to device
output: (Continued)
V_G Feedthrough
Application Information
THEORY OF OPERATION
The LMH6503 is a linear wideband variable-gain amplifier as
illustrated in Figure 1. A voltage input signal may be applied
differentially between the two inputs (+V_IN, −V_IN), or single-
endedly by grounding one of the two unused inputs. The
LMH6503 input buffers convert the input voltage to a current
(I_RG) that is a function of the differential input voltage (V_INPUT
= (+V_IN) - (−V_IN)) and the value of the gain setting resistor
(R_G). This current (I_RG) is then mirrored to a gain stage with
a current gain of K (1.72 nominal). The voltage controlled
two-quadrant multiplier attenuates this current which is then
converted to a voltage via the output amplifier. This output
amplifier is a current feedback op amp configured as a
Transimpedance amplifier. Its Transimpedance gain is the
feedback resistor (R_F). The input signal, output, and gain
control are all voltages. The output voltage can easily be
calculated as shown in Equation 1:
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(3)
Notice also that Equation 3 holds for both differential and
single ended operation.
(1)
Where K = 1.72 (Nominal)
since:
The gain of the LMH6503 is therefore a function of three
external variables: R_G, R_F, and V_G as expressed in Equation
2:
(2)
The gain control voltage (V_G) has an ideal input range of −1V
< V_G < +1V. At V_G = +1V, the gain of the LMH6503 is at its
maximum as expressed in Equation 3:
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FIGURE 1. LMH6503 Functional Block Diagram
CHOOSING R_F AND R_G
R_G is calculated from Equation 4. V_INPUTMAX is the maxi-
mum peak
(4)
input voltage (V_pk) determined by the application. I_RGMAX is
the maximum allowable current through R_G and is typically
2.3mA. Once A_VMAX is determined from the minimum input
and desired output voltages, R_F is then determined using
Equation 5. These values of R_F and R_G are
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