9
LT5537
5537fa
APPLICATIO S I FOR ATIO
WUUU
Dynamic Range
The LT5537 is capable of detecting and log-converting an
input signal over a wide dynamic range. The range of the
output voltage may be limited, however, and the monoto-
nicity of the output versus input at high input level may be
affected if the supply voltage is low and the log-linear slope
is set too high. The minimum V
CC
to support 90dB
dynamic range with 20mV/dB slope is 2.8V under nominal
conditions at 25°C. The data shown in the Typical Perfor-
mance Characteristics plots was taken with V
CC
= 3V. If
there is difficulty encountered in achieving the desired
dynamic range, then the user is advised to increase the
supply voltage or else to decrease the output slope by
connecting a smaller valued resistor between the output
and ground.
7k
TO 2ND
STAGE
CAP
–
IN
+
V
BIAS
5537 F04
IN
–
V
CC
7k
CAP
+
Figure 3. Simplified Input Circuit
Figure 4. Input Admittance
Figure 5. Differential Input Matching to 200Ω
Input Matching
The LT5537 has a high impedance input (Figure 3). The
differential input impedance is derived from S11 measure-
ment with one of the input pins AC grounded (Figure 4). At
200MHz, the input is equivalent to 1.73k//1.45pF (Table 1).
The input dynamic range is constant in voltage terms,
ranging from approximately –89dBVrms to 1dBVrms at
200MHz. The dynamic range expressed in power is
depen
dent on the actual impedance selected in the ap-
plication design.
Table 1. Parallel Equivalent RC of the LT5537 Input
FREQUENCY R C
100MHz 1.85kΩ 1.51pF
200MHz 1.73kΩ 1.45pF
400MHz 1.07kΩ 1.48pF
600MHz 673Ω 1.52pF
800MHz 435Ω 1.65pF
1000MHz 303Ω 1.78pF
The simplest way of input matching the LT5537 is to
terminate the input signal with a 50Ω resistor and AC
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
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
ETC4-1-2
(1:4)
N/C
IN
+
IN
–
C1
C2
R2
240Ω
J1
INPUT
5537 F06
2
3
