³

2

dt

2

dt =

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LMH2121

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SNVS876A – AUGUST 2012 – REVISED MARCH 2013

(7)

current is isolated and integrated. The input of amplifier A acts as the nulling point of the negative feedback loop,

yielding:

(8)

which implies that the average power content of the current related to the output voltage of the LMH2121 is

made equal to the average power content of the current related to the RF input signal.

For a negative-feedback system, the transfer function is given by the inverse function of the feedback block.

Therefore, to have a linear conversion gain for this RF detector, the feedback network implements a linear

function as well resulting in an overall transfer function for the LMH2121 of:

(9)

in which k is the conversion gain. Note that as a result of the feedback loop the square root is implemented.

The envelope response time of this fast-responding RF detector is given by the gain-bandwidth product of the

feedback loop.

Given this architecture for the RF detector, the high performance of the LMH2121 can be understood. In theory

the accuracy of the linear transfer function is set by:

•

The linear feedback network, which basically needs to process a DC signal only.

•

A high loop gain for the feedback loop, which is ensured by the high amplifier gain A.

The square-root functionality is inherent to the feedback loop and the use of a multiplier. Therefore, a very

accurate relation between the power content of the input signal and the output is obtained.

RF Input and Enable

To minimize pin-count, in this case. only 4, the RF input and the enable functionality are combined into one pin.

RF signal and a DC path for the enable voltage. Care should be taken with the selection of capacitor C. The turn-

on time of the RF detector will increase when a large capacitor value is chosen. This is because the capacitor

recommended which hardly impacts the turn-on time for those values. The turn-on time is mainly determined by

the device itself.

RF systems typically use a characteristic impedance of 50

Ω; the LMH2121 is no exception to this. The 50Ω input

impedance enables an easy, direct connection to a directional coupler without the need for impedance

adjustments. Please note that as a result of the internal AC coupling the 50ohm is not obtained for the complete

DC to HF range. However, the input impedance does approximate 50

Ω at the usual transmit bands.

The LMH2121 can be brought into a low power consumption shutdown mode by means of the DC enable level

switched to high impedance.

Output

The output of the LMH2121 provides a DC voltage that is a measure for the applied RF power to the input pin. It

tracks the input RF envelope with a 3 dB bandwidth around 2 MHz. The output voltage has a linear-in-V

response for an applied RF signal. In active mode the output impedance is 100

Ω such that with an external

capacitor some filtering can be obtained if necessary. The output impedance of the LMH2121 is high impedance

in shutdown. This enables a parallel connection of multiple detector outputs where one of the detectors is

enabled at a time.

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