LMH6503 datasheet(14/20 Pages) NSC

14 / 20 page

Typical Performance Charateristics Unless otherwise specified: V

S = ±5V, 25˚C, VG =VG_MAX,VCM

= 0V, R

F =1k

Ω,R

G = 174

Ω, both inputs terminated in 50Ω,R

L = 100

Ω, Typical values, results referred to device

output: (Continued)

G Feedthrough

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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,−VIN), or single-

endedly by grounding one of the two unused inputs. The

LMH6503 input buffers convert the input voltage to a current

RG) that is a function of the differential input voltage (VINPUT

= (+V

IN) - (−VIN)) and the value of the gain setting resistor

G). This current (IRG) 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:

(1)

Where K = 1.72 (Nominal)

since:

The gain of the LMH6503 is therefore a function of three

external variables: R

G,RF, and VG as expressed in Equation

(2)

The gain control voltage (V

G) has an ideal input range of −1V

< V

G < +1V. At VG = +1V, the gain of the LMH6503 is at its

maximum as expressed in Equation 3:

(3)

Notice also that Equation 3 holds for both differential and

single ended operation.

CHOOSING R

F AND RG

G is calculated from Equation 4. VINPUTMAX is the maxi-

mum peak

(4)

input voltage (V

pk) determined by the application. IRGMAX 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 RG are

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FIGURE 1. LMH6503 Functional Block Diagram

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