ADL5904ACPZN-R7 datasheet(22/27 Pages) AD

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ADL5904

Data Sheet

Rev. B | Page 22 of 27

CH2 1.0V

M10ns 20GS/s

A CH2

1.36V

Ω

CH3 100mV

Ω

Figure 52. Q Output Response, PIN = Off to −7 dBm, Overdrive Threshold

Voltage Set to Trigger at −10 dBm (Overdrive Level = 3 dB)

SETTING THE VIN− THRESHOLD DETECTION

VOLTAGE

Figure 53 shows the typical relationship between the voltage on

the VIN− pin and the resulting RF power threshold that causes

Q and Q to latch high and low, respectively. This data is also

presented in Table 6.

0.001

0.01

0.1

–40 –35 –30 –25 –20 –15 –10

–5

PIN (dBm)

0.01GHz

0.03GHz

0.1GHz

0.9GHz

1.9GHz

2.6GHz

3.6GHz

5.8GHz

Figure 53. VIN− Threshold Voltage vs. PIN at Various Frequencies

Use Figure 53 and Table 6 to set the threshold voltage on the

VIN− pin. However, because the relationship between the

threshold voltage on VIN− and the resulting RF threshold

power varies from device to device, there is an error level of up

to ±2.5 dB. For example, if the voltage on VIN− is set to cause

the circuit to trip when the input power exceeds 0 dBm at 900 MHz

(VIN− = 241 mV from Table 6), the trip point can vary from

device to device by ±2.5 dB at frequencies at or above 100 MHz

and +2.5 dB to −5.5 dB for frequencies below 100 MHz. In Table 6,

no recommended voltages are provided for input power levels

below −25 dBm from 10 MHz to 3.5 GHz and below −20 dBm

at 5.8 GHz. This is as a result of the increased temperature drift

at these input power levels. Likewise, from 10 MHz to 3.5 GHz,

no recommended voltages are provided for input power levels

above 13 dBm because, at this power level, the response of the

ADL5904 starts to become more nonlinear.

To set the threshold detect level more precisely, there are two

calibration options. A single-point calibration is easily

accomplished by applying the threshold trip power level and

then adjusting VIN− until Q trips high. Initially, set VIN− to a high

level such as 2 V, and then assert RST high and back to low to

ensure that Q is low. Next, apply the RF input threshold power

level to RFIN. Then, reduce the voltage on VIN− until the Q

output goes high. Use this resulting voltage to set the threshold

level when the equipment is in operation.

Alternatively, by measuring the voltage on the VCAL output pin

with and without RF power applied, an equation can be derived

that establishes a precise relationship between the VIN− voltage

and the associated RF input power trip point.

Within the linear operating range of the ADL5904, there is a

linear relationship between VCAL − VCALOFF and the input

voltage on RFIN.

VCAL − VCALOFF = Slope × (VRFIN − Intercept)

(6)

where:

VCAL is the measured output voltage on the VCAL pin.

VCALOFF is the measured output voltage on the VCAL pin with

no RF input signal applied.

VRFIN is the RF input power (in dBm) converted into volts rms,

that is,

log













−

RFIN

(8)

where:

R is the characteristic impedance (usually 50 Ω).

PIN is the input power in dBm.

Rewriting the equation results in













−













−

Intercept

Slope

CALOFF

CAL

log

(9)

The voltage that must be applied to the VIN− pin for a particular

input power is equal to (VCAL − VCALOFF). Therefore,

Equation 9 can be rewritten as













−













−

Intercept

Slope

VIN

log

(10)

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