AD8305 Analog Devices, AD8305 Datasheet - Page 10

• Manufacturer Part Number: AD8305
• Description: 100 Db-range (10nA-1mA) Logarithmic Converter
• Manufacturer: Analog Devices

AD8305
Managing Intercept and Slope
When using a single supply, VRDZ should be directly connected
to VREF to allow operation over the entire five-decade input
current range. As noted previously, this introduces an accurate
offset voltage of 0.8 V at the VLOG pin, equivalent to four decades,
resulting in a logarithmic transfer function that can be written as:
where I
Thus, the effective intercept current I
thousandth of I
recommended value of I
The slope can be reduced by attaching a resistor to the VLOG
pin. This is strongly discouraged, in view of the fact that the
on-chip resistors will not ratio correctly to the added resistance.
Also, it is rare that one would want to lower the basic slope of
10 mV/dB; if this is needed, it should be effected at the low
impedance output of the buffer, which is provided to avoid such
miscalibration and also allow higher slopes to be used.
The AD8305 buffer is essentially an uncommitted op amp with
rail-to-rail output swing, good load-driving capabilities and a
unity-gain bandwidth of >12 MHz. In addition to allowing the
introduction of gain, using standard feedback networks and
thereby increasing the slope voltage V
to implement multipole low-pass filters, threshold detectors,
and a variety of other functions. Further details of these can be
found in the AD8304 data sheet.
Response Time and Noise Considerations
The response time and output noise of the AD8305 are funda-
mentally a function of the signal current I
the bandwidth is proportional to I
output low frequency voltage-noise spectral-density is a function
of I
Details of the noise and bandwidth performance of translinear
log amps can be found in the AD8304 Data Sheet.
APPLICATIONS
The AD8305 is easy to use in optical supervisory systems and in
similar situations where a wide ranging current is to be converted
to its logarithmic equivalent, which is represented in decibel
terms. Basic connections for measuring a single-current input are
shown in Figure 2, which also includes various nonessential com-
ponents, as will be explained.
PD
V
(TPC 15) and also increases for small values of I
LOG
INTC
=
=
= I
V
V
Y
Y
REF
REF
log
log
/10
, corresponding to 1 nA when using the
10
10
4
(
(
10
I
PD
REF
4
/
¥
I
= 10 mA.
INTC
I
PD
/
)
I
PD
REF
, as shown in TPC 13. The
Y
)
, the buffer can be used
INTC
PD
. For small currents,
is only one ten-
REF
.
(6)
–10–
The 2 V difference in voltage between the VREF and INPT pins
in conjunction with the external 200 kW resistor R
reference current I
VRDZ to VREF raises the voltage at VLOG by 0.8 V, effectively
lowering the intercept current I
it at 1 nA. A wide range of other values for I
100 nA to over 1 mA, may be used. The effect of such changes
is shown in TPC 3.
Any temperature variation in R
when estimating the stability of the intercept. Also, the overall
noise will increase when using very low values of I
intercept applications, there is little benefit in using a large
reference current, since this only compresses the low current
end of the dynamic range when operated from a single supply,
here shown as 5 V. The capacitor between VSUM and ground
is recommended to minimize the noise on this node and to help
provide a clean reference current.
Since the basic scaling at VLOG is 0.2 V/decade, and thus a swing
of 4 V at the buffer output would correspond to 20 decades, it will
often be useful to raise the slope to make better use of the rail-
to-rail voltage range. For illustrative purposes, the circuit in
Figure 2 provides an overall slope of 0.5 V/decade (25 mV/dB).
Thus, using I
to 1.4 V at I
3.5 V, corresponding to a dynamic range of 120 dB (electrical,
that is, 60 dB optical power).
The optional capacitor from VLOG to ground forms a single-pole
low-pass filter in combination with the 4.55 kW resistance at this
pin. For example, using a C
frequency is 3.5 kHz. Such filtering is useful in minimizing the
output noise, particularly when I
more effective in reducing the total noise; examples are provided
in the AD8304 data sheet.
1k
V
Figure 2. Basic Connections for Fixed Intercept Use
200k
BIAS
1nF
I
PD
VRDZ
VREF
VSUM
1k
1nF
IREF
INPT
PD
1nF
REF
= 1 mA while the buffer output runs from 0.5 V to
0.5V
= 10 mA, V
REF
0.5V
of 10 mA into Pin IREF. Connecting pin
20k
COMM
VNEG
Q2
Q1
LOG
80k
FLT
INTC
REF
–
+
V
runs from 0.2 V at I
V
PD
2.5V
of 10 nF, the –3 dB corner
VPOS
BE2
BE1
COMPENSATION
must be taken into account
TEMPERATURE
by a factor of 104 to position
is small. Multipole filters are
+5V
GENERATOR
BIAS
COMM
REF
6.69k
14.2k
I
LOG
, from under
REF
REF
0.5 log
451
provide a
PD
COMM
. In fixed-
VOUT
SCAL
BFIN
10
= 10 nA
REV. A
( )
1nA
I
PD
12k
8k
VLOG
10nF
C
FLT