ADL5310ACP-R2 datasheet(12/20 Pages) AD

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ADL5310

Rev. A | Page 12 of 20

The voltage VLOG is generated by applying ILOG to an internal

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 ILOG generates a voltage of

VLOG = ILOG × 4.55 kΩ

= 44 µA × 4.55 kΩ × log10(IPD/IINTC)

(5)

= VY log10(IPD/IINTC)

where VY = 200 mV/decade or 10 mV/dB. Note that any resis-

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.

VLOG may also swing below ground when dual supplies (VP and

VN) are used. When VN = −0.5 V or larger, the input Pins INP1

(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

VLOG = VY log10(104 × IPD/IREF)

= VY log10(IPD/IINTC)

(6)

where IINTC = IREF/104.

Thus, the effective intercept current IINTC is only one ten-

thousandth of IREF, corresponding to 300 pA when using the

recommended value of IREF = 3 µA.

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 VY, the buffer can be used to implement multipole, low-

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, IPD. For small currents,

the bandwidth is proportional to IPD, as shown in Figure 15. The

output low frequency voltage-noise spectral-density is a

function of IPD (see Figure 17) and also increases for small

values of IREF. Details of the noise and bandwidth performance

of translinear log amps can be found in the AD8304 data sheet.

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