ADL5310-EVAL AD [Analog Devices], ADL5310-EVAL Datasheet - Page 12

• Manufacturer Part Number: ADL5310-EVAL
• Description: 120 dB Range (3 nA to 3 mA) Dual Logarithmic Converter
• Manufacturer: AD [Analog Devices]

ADL5310
The voltage V
resistance of 4.55 kΩ, formed by the parallel combination of a
6.69 kΩ resistor to ground and a 14.2 kΩ resistor to Pin VRDZ
(typically tied to the 2.5 V reference, VREF). At the LOG1
(LOG2) pin, the output current I
where V
tive loading on LOG1 (LOG2) lowers this slope and results in
an overall scaling uncertainty. This is due to the variability of
the on-chip resistors compared to the off-chip load. As a con-
sequence, this practice is not recommended.
V
V
(INP2) and IRF1 (INP2) may be positioned at ground level
simply by grounding VSUM. Care must be taken to limit the
power consumed by the input BJT devices when using a larger
negative supply, because self-heating degrades the accuracy at
higher currents.
MANAGING INTERCEPT AND SLOPE
When using a single supply, VRDZ should be directly connected
to VREF to allow operation over the entire 6-decade input
current range. As noted in the Theory section, this introduces
an accurate offset voltage of 0.8 V at the LOG1 and LOG2 pins,
equivalent to four decades, resulting in a logarithmic transfer
function that can be written as
where I
LOG
N
) are used. When V
V
= 44 µA × 4.55 kΩ × log
= V
V
= V
may also swing below ground when dual supplies (V
LOG
LOG
INTC
Y
Y
Y
= I
= 200 mV/decade or 10 mV/dB. Note that any resis-
= V
log
log
= I
LOG
10
10
Y
LOG
REF
(I
(I
log
× 4.55 kΩ
PD
PD
/10
is generated by applying I
/I
/I
10
(10
INTC
INTC
4
.
N
4
)
)
= −0.5 V or larger, the input Pins INP1
× I
PD
/I
10
(I
REF
PD
LOG
)
/I
INTC
generates a voltage of
)
LOG
to an internal
(5)
(6)
P
and
Rev. A | Page 12 of 20
Thus, the effective intercept current I
thousandth of I
recommended value of I
The slope can be reduced by attaching a resistor between the log
amp output pin, LOG1 or LOG2, and ground. This is strongly
discouraged given that the on-chip resistors do not ratio
correctly to the added resistance. Also, it is rare that one would
wish 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
amps, which are provided to avoid such miscalibration and to
allow higher slopes to be used.
Each of the ADL5310’s buffers is essentially an uncommitted
operational amplifier with rail-to-rail output swing, good load-
driving capabilities, and a typical unity-gain bandwidth of
15 MHz. In addition to allowing the introduction of gain, using
standard feedback networks and thereby increasing the slope
voltage V
pass filters, threshold detectors, and a variety of other functions.
Further details on these applications can be found in the
AD8304 data sheet.
RESPONSE TIME AND NOISE CONSIDERATIONS
The response time and output noise of the ADL5310 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
values of I
of translinear log amps can be found in the AD8304 data sheet.
Y
, the buffer can be used to implement multipole, low-
REF
PD
. Details of the noise and bandwidth performance
(see Figure 17) and also increases for small
REF
, corresponding to 300 pA when using the
REF
= 3 µA.
PD
, as shown in Figure 15. The
INTC
is only one ten-
PD
. For small currents,