Datasheet LTC5536 (Analog Devices) - 8

LTC5536
U U W U APPLICATIO S I FOR ATIO Operation
operation of the comparator (no latching action), the LEN The LTC5536 is configured as a fast detector and high pin should be connected to ground. speed comparator for RF power detection and RF power The comparator output (VOUT) rise and fall times are alarms. The product integrates several functions to pro- approximately 2ns (unloaded). The propagation delay for vide RF power detection over frequencies ranging from the comparator alone was characterized by applying a 600MHz to 7GHz. These functions include an RF Schottky continuous 2GHz RF signal to the RFIN input. Then a 1MHz diode peak detector, a level shift amplifier to convert the RF square wave (0V to 2.5V) was applied to the VM input to input signal to low frequency, and a fast comparator. The switch the comparator. Note that there is a signal inver- LTC5536 provides a comparator reference input VM and a sion, because the VM pin is connected internally to the latch enable input LEN. negative comparator input. The time delay from the tran- sition edge of the square wave at the VM input to the
RF Detector
corresponding VOUT output transition (rising or falling) is The internal RF Schottky diode peak detector and level shown in Table 1. shift amplifier convert the RF input signal to a low fre-
Table 1. Comparator Propagation Delay
quency signal. The detector demonstrates excellent effi- RF Input VOUT Rising VOUT Falling ciency and linearity over a wide range of input power. The Level Edge Delay Edge Delay Schottky diode is biased at about 55µA, and drives a 15pF (dBm) (ns) (ns) internal peak detector capacitor. –10 38.5 36 0 24 40
High Speed Comparator
10 20 86 The fast internal comparator compares the external refer- ence voltage at VM to the internal signal voltage from the
Overall Propagation Delay and Response Time
peak detector, and produces the output signal, VOUT. The Figure 2 shows measurements of total propagation delay internal peak detector voltage is factory trimmed to 100mV from the RF with no RF signal present. The comparator has approxi- IN signal input to the VOUT output of the LTC5536, plotted as a function of RF input power. The mately 10mV of hysteresis, with a typical VOUT low-to-high response is shown for RF Signal Absent-to-RF Signal switching point of 100mV and a VOUT high-to-low switch- Present Transitions (Rising Edge V ing point of 90mV with no RF signal present. OUT), and for RF Signal Present-to-RF Signal Absent Transitions (Falling Edge The comparator also has a built-in latch. This will cause the VOUT). The LTC5536’s RF detector is optimized as a VOUT output to latch high on a positive comparator transi- positive peak detector. Consequently, the device responds tion (increasing RF power), when the LEN pin is high, as to a rising signal at the RF input much more rapidly than indicated in the waveforms of Figure 1. For transparent to a falling signal. Correspondingly, Rising Edge VOUT transitions are much more rapid than Falling Edge transi- EXTERNAL LEN tions, as shown in Figure 2. The minimum propagation ENABLE delay is about 20ns at room temperature, in response to strong overdrive conditions at the RF OUTPUT IN input. These VM RF DET V results were measured by applying a 1GHz RF signal that P was amplitude modulated by a 1MHz square wave with V OUTPUT OF OUT COMPARATOR 50% duty cycle. An example time domain waveform is shown in Figure 3. VOUT VOUT VOUT TRANSPARENT LATCHED TRANSPARENT 5536 AI01
Figure 1. LTC5536 LEN Function Waveform
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