Datasheet LT5537 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | Wide Dynamic Range RF/IF Log Detector |
| Pages / Page | 16 / 9 — APPLICATIO S I FOR ATIO. Dynamic Range. Input Matching. Table 1. Parallel … |
| File Format / Size | PDF / 315 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. Dynamic Range. Input Matching. Table 1. Parallel Equivalent RC of the LT5537 Input. FREQUENCY
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LT5537
U U W U APPLICATIO S I FOR ATIO Dynamic Range Input Matching
The LT5537 is capable of detecting and log-converting an The LT5537 has a high impedance input (Figure 3). The input signal over a wide dynamic range. The range of the differential input impedance is derived from S11 measure- output voltage may be limited, however, and the monoto- ment with one of the input pins AC grounded (Figure 4). At nicity of the output versus input at high input level may be 200MHz, the input is equivalent to 1.73k//1.45pF (Table 1). affected if the supply voltage is low and the log-linear slope The input dynamic range is constant in voltage terms, is set too high. The minimum VCC to support 90dB ranging from approximately –89dBVrms to 1dBVrms at dynamic range with 20mV/dB slope is 2.8V under nominal 200MHz. The dynamic range expressed in power is conditions at 25°C. The data shown in the Typical Perfor- dependent on the actual impedance selected in the ap- mance Characteristics plots was taken with VCC = 3V. If plication design. there is difficulty encountered in achieving the desired dynamic range, then the user is advised to increase the
Table 1. Parallel Equivalent RC of the LT5537 Input
supply voltage or else to decrease the output slope by
FREQUENCY R C
connecting a smaller valued resistor between the output 100MHz 1.85kΩ 1.51pF and ground. 200MHz 1.73kΩ 1.45pF VCC 400MHz 1.07kΩ 1.48pF 600MHz 673Ω 1.52pF CAP+ CAP– TO 2ND 800MHz 435Ω 1.65pF 7k 7k STAGE 1000MHz 303Ω 1.78pF IN+ IN– The simplest way of input matching the LT5537 is to 5537 F04 terminate the input signal with a 50Ω resistor and AC VBIAS couple it to one of the input pins while AC grounding the other input pin (Figure 13). The sensitivity (defined as the minimum input power required for the output to be within 3dB of the ideal log-linear response) is –76.4dBm at
Figure 3. Simplified Input Circuit
200MHz in this case. To achieve the best sensitivity, the input termination impedance should be increased and the input pins should be differentially driven. An example application circuit is shown in Figure 5 which uses a transformer to step up the impedance and perform the balun function. The 240Ω resistor (R2) sets the impedance at the input of the chip to 200Ω. A 1:4 transformer is used to match the 50Ω signal source impedance to the circuit input impedance. C1 and C2 are DC blocking capacitors. This application circuit has a (3dB error) sensitivity of –82.4dBm at 200MHz. M/A-COM C1 IN+ ETC4-1-2 J1 2 INPUT R2 N/C 240Ω C2 IN– 3 (1:4) 5537 F06
Figure 4. Input Admittance Figure 5. Differential Input Matching to 200
Ω 5537fa 9
U U W U APPLICATIO S I FOR ATIO Dynamic Range Input Matching
The LT5537 is capable of detecting and log-converting an The LT5537 has a high impedance input (Figure 3). The input signal over a wide dynamic range. The range of the differential input impedance is derived from S11 measure- output voltage may be limited, however, and the monoto- ment with one of the input pins AC grounded (Figure 4). At nicity of the output versus input at high input level may be 200MHz, the input is equivalent to 1.73k//1.45pF (Table 1). affected if the supply voltage is low and the log-linear slope The input dynamic range is constant in voltage terms, is set too high. The minimum VCC to support 90dB ranging from approximately –89dBVrms to 1dBVrms at dynamic range with 20mV/dB slope is 2.8V under nominal 200MHz. The dynamic range expressed in power is conditions at 25°C. The data shown in the Typical Perfor- dependent on the actual impedance selected in the ap- mance Characteristics plots was taken with VCC = 3V. If plication design. there is difficulty encountered in achieving the desired dynamic range, then the user is advised to increase the
Table 1. Parallel Equivalent RC of the LT5537 Input
supply voltage or else to decrease the output slope by
FREQUENCY R C
connecting a smaller valued resistor between the output 100MHz 1.85kΩ 1.51pF and ground. 200MHz 1.73kΩ 1.45pF VCC 400MHz 1.07kΩ 1.48pF 600MHz 673Ω 1.52pF CAP+ CAP– TO 2ND 800MHz 435Ω 1.65pF 7k 7k STAGE 1000MHz 303Ω 1.78pF IN+ IN– The simplest way of input matching the LT5537 is to 5537 F04 terminate the input signal with a 50Ω resistor and AC VBIAS couple it to one of the input pins while AC grounding the other input pin (Figure 13). The sensitivity (defined as the minimum input power required for the output to be within 3dB of the ideal log-linear response) is –76.4dBm at
Figure 3. Simplified Input Circuit
200MHz in this case. To achieve the best sensitivity, the input termination impedance should be increased and the input pins should be differentially driven. An example application circuit is shown in Figure 5 which uses a transformer to step up the impedance and perform the balun function. The 240Ω resistor (R2) sets the impedance at the input of the chip to 200Ω. A 1:4 transformer is used to match the 50Ω signal source impedance to the circuit input impedance. C1 and C2 are DC blocking capacitors. This application circuit has a (3dB error) sensitivity of –82.4dBm at 200MHz. M/A-COM C1 IN+ ETC4-1-2 J1 2 INPUT R2 N/C 240Ω C2 IN– 3 (1:4) 5537 F06
Figure 4. Input Admittance Figure 5. Differential Input Matching to 200
Ω 5537fa 9
