ADL5315ACPZ-WP AD [Analog Devices], ADL5315ACPZ-WP Datasheet - Page 12

• Manufacturer Part Number: ADL5315ACPZ-WP
• Description: Precision Wide Range (3 nA to 3 mA) High-Side Current Mirror
• Manufacturer: AD [Analog Devices]

ADL5315
TRANSLINEAR LOG AMP INTERFACING
The mirror current output, IOUT, of the ADL5315 is designed
to interface directly to an Analog Devices translinear
logarithmic amplifier, such as the AD8304, AD8305, or
ADL5306.
Figure 24 shows the basic connections necessary for interfacing
the ADL5315 to the AD8305. In this configuration, the designer
can use the full current mirror range of the ADL5315 for high
accuracy power monitoring.
The measured rms noise voltage at the output of the AD8305 vs.
the input current is shown in Figure 23, both for the AD8305 by
itself and in cascade with the ADL5315. The relatively low noise
produced by the ADL5315, combined with the additional noise
filtering inherent in the frequency response characteristics of
the AD8305, results in minimal degradation to the noise
performance of the AD8305.
C
PIN
SET
TIA
4
3
2
1
COMM
SREF
VSET
INPT
20kΩ
REFERENCE
VOLTAGE
I
PD
DATA PATH
CURRENT
MIRROR
Figure 24. Interfacing the ADL5315 to the AD8305 for High Accuracy PIN Power Monitoring
CURRENT
LIMITING
1:1
ADL5315
I
PD
VPOS
RLIM
IOUT
NC
5
6
7
8
R
I
LIM =
LIM
COMPENSATION
AD8305 INPUT
= 48V
NETWORK
1mA – 16mA
1nF
I
R
1kΩ
LIM
LIM
– 3kΩ
Rev. 0 | Page 12 of 20
V
POS
4.7nF
2kΩ
200kΩ
Careful consideration should be made to the layout of the
circuit board in this configuration. Leakage current paths in the
board itself could lead to measurement errors at the output of
the translinear log amp, particularly when measuring the low
end of the ADL5315’s dynamic range. It is recommended that
when designing such an interface that a guard potential be used
to minimize this leakage. This can be done by connecting the
translinear log amp’s VSUM pin to the NC pin of the ADL5315,
with the VSUM guard trace running on both sides of the IOUT
trace. Additional details on using VSUM can be found in the
AD8304 or AD8305 data sheets. The VSET pin of the ADL5315
can be used in a similar fashion to guard the INPT trace.
1
2
3
4
5.5m
5.0m
4.5m
4.0m
3.5m
3.0m
2.5m
2.0m
1.5m
1.0m
0.5m
0.1μF
VRDZ
VREF
IREF
INPT
0
1n
Figure 23. Measured RMS Noise of AD8305 vs. AD8305
AD8305
10n
AD8305 ONLY
3V TO 12V
Cascaded with ADL5315
100n
VLOG
VOUT
SCAL
BFIN
AD8305 AND
ADL5315
I
INPT
12
11
10
9
1μ
(A)
OUTPUT
V
OUT
10μ
= 0.2 × LOG
100μ
10
(I
PDM
/1nA)
1m