MAX4207
Precision Transimpedance Logarithmic
Amplifier with Over 5 Decades of Dynamic Range
______________________________________________________________________________________ 11
Substituting into the TE approximation,
TE ≅ ± (1V/decade)(0.05 log
10
(100µA/100nA)
±4 (±5mV ±2mV) = ±[0.15V ±4(±7mV)]
As a worst case, one finds TE ≅ ±178mV or ±3.6% of
full scale.
When expressed as a voltage, TE increases in proportion
with an increase in gain as the contributing errors are
defined at a specific gain. Calibration using a look-up
table eliminates the effects of gain and output offset
errors, leaving conformity error as the only factor
contributing to total error. For further accuracy, consider
temperature monitoring as part of the calibration process.
Applications Information
Input Current Range
Five decades of input current across a 10nA to 1mA
range are acceptable for I
LOG
and I
REF
. The effects of
bias currents increase as I
LOG
and I
REF
fall below
10nA. Bandwidth decreases at low I
LOG
values (see
the Frequency Response and Noise Considerations
section). As I
LOG
and I
REF
increase to 1mA or higher,
transistors become less logarithmic in nature. The
MAX4207 incorporates leakage current compensation
and high-current correction circuits to compensate for
these errors.
Frequency Compensation
The MAX4207’s frequency response is a function of the
input current magnitude and the selected compensation
network at LOGIIN and REFIIN. The compensation net-
work comprised of C
COMP
and R
COMP
ensures stability
over the specified range of input currents by introducing
an additional pole/zero to the system. For the typical
application, select C
COMP
= 32pF and R
COMP
= 330Ω.
Frequency Response and Noise Considerations
The MAX4207 bandwidth is proportional to the magnitude
of the I
REF
and I
LOG
currents, whereas the noise is
inversely proportional to I
REF
and I
LOG
currents.
Common Mode
A 0V common-mode input voltage, V
CMVOUT
, is avail-
able at CMVOUT and can be used to bias the logging
and reference amplifier inputs by connecting CMVOUT
to CMVIN. A voltage between 0 and 0.5V, connected to
CMVIN, may be used to bias the logging and reference
transistor collectors, thereby optimizing performance.
Adjusting the Logarithmic Intercept
Adjust the logarithmic intercept by changing the refer-
ence current, I
REF
. A resistor from REFISET to GND
(see Figure 5) adjusts the reference current, according
to the following equation:
where V
REFISET
is 0.5V. Select R
SET
between 5kΩ and
5MΩ. REFIOUT current range is 10nA to 10µA only.
Dual-Supply Operation
The MAX4207 operates only from dual ±2.7 to ±5.5V sup-
plies. The relationship of inputs to outputs is a function of
I
REF
, relative to I
LOG
, and the configuration of the uncom-
mitted amplifier. The uncommitted amplifier can be con-
figured in either inverting or noninverting mode. In an
inverting configuration, the uncommitted amplifier output,
LOGV2, is positive and LOGV1 is negative when I
LOG
exceeds I
REF
. When operating in a noninverting configu-
ration, LOGV2 and LOGV1 are both negative when I
LOG
exceeds I
REF
(see Table 1). An inverting configuration of
the uncommitted buffer is recommended when large out-
put offset voltage adjustments are required using OSADJ.
By connecting CMVOUT and CMVIN, the log and refer-
ence amplifier inputs (LOGIIN and REFIIN) are biased at
0V. Applying the external voltage (0 to 0.5V) to CMVIN
optimizes the application’s performance.
