ADL5304ACPZ-R2 datasheet(17/32 Pages) AD

17 / 32 page

Data Sheet

ADL5304

Rev. A | Page 17 of 32

THEORY OF OPERATION

BASIC CONCEPTS

The ADL5304 exploits the logarithmic relationship between base

emitter voltage, VBE, and the collector current, IC, of a bipolar

junction transistor (see Equation 5). This is the fundamental basis

of the extended class of translinear circuits. A log amp based on this

unique property of the bipolar transistor is called a translinear log

amp to distinguish it from log amps designed for RF applications,

which use different principles while having similar objectives.

VBE = VT log(IC/IS)

(5)

Two scaling quantities appear in Equation 5: the thermal voltage,

VT = kT/q, and the saturation current, IS. The thermal voltage is

of crucial importance in determining the logarithmic slope in a

translinear log amp. VT has a process invariant value of 25.69 mV

at T = 25°C and varies in proportion to the absolute temperature

(PTAT). Saturation current, unlike VT, is a process and device

dependent parameter. Saturation current is typically approximately

10−16A at 25°C, but exhibits enormous variation over temperature,

by a factor of more than a billion.

The temperature dependence of saturation current is compensated

in the ADL5304 by using a second reference transistor, having

an identical variation, to stabilize the intercept by using the

difference between the two VBEs.

Input currents, INUM and IDEN, are the numerator and denominator

of the logarithmic argument that follows:

ΔVBE = VT log (INUM/IDEN)

(6)

In log ratio applications, both INUM and IDEN may each vary over

the full specified range of 1 pA to 10 mA. However, in default

operation, IDEN takes the internally preset current of IREF = 100 nA.

Equation 6 shows that the ΔVBE is still PTAT, but the required

logarithmic slope must be temperature stable; therefore, this is

corrected using proprietary circuit techniques. Using this

correction the relationship between a photodiode current, IPD,

applied to INUM, and the voltage appearing at the output at

VLOG is

VLOG = VY log10(IPD/IZ)

(7)

where:

VY is the log slope voltage (and, for the case of base-10 logarithms,

it is also the volts per decade ).

IZ is the extrapolated log intercept.

The relationship between VY and ΔVBE is a factor close to 3.333

in the default configuration from (VNUM − VDEN) to the output of

VLOG. Because a decade change in the input current ratio results

in close to a 60 mV/decade change in ΔVBE; multiplying this by

3.333 results in 0.2 V/decade. During fabrication, VY is trimmed

to 0.2 V/decade (10 mV/dB), IREF to 100 nA, VOFS to 1.500 V,

and IZ to 3.162 fA. When IPD = 1 pA, the output VLOG has a value

of 0.5 V (see Figure 44). IZ is small because VLOG is always above

ground potential even at the lowest end of the dynamic range,

when using VOFS = 1.500 V. If a negative supply is used, this

voltage can cross zero at the intercept value.

The output for the value of IPD can be calculated using Equation 8.

For example, with an input current of 100 nA,

VLOG = 0.2 V log10(100 nA/3.162 fA) = 1.500 V

(8)

The slope and intercept can be adjusted to suit the application,

to either higher or lower values, without significant loss of

calibration accuracy.

OPTICAL MEASUREMENTS

It is important to understand the transducer aspects of a photo-

diode when interpreting the photodiode current relative to the

incident optical power.

In purely electrical circuits, current applied to a resistive load

results in a power proportional to the square of the current. For

a photodiode interface, however, there is a difference in scaling

because photon-generated photodiode current (IPD) flows in an

element biased at a fixed voltage. IPD is equal to the optical

power (POPT) absorbed in the detector times the responsivity of

the photodiode (ρ).

IPD = ρ × POPT

(9)

A similar relationship exists between the intercept current, IZ,

and effective intercept power, PZ.

IZ = ρ × PZ

(10)

Therefore, the VOUT equation for the ADL5304 may be written as

VLOG = VYlog10(POPT/PZ)

(11)

For the ADL5304 operating in its default mode, an IZ of 3.162 fA

corresponds to a PZ of 3.95 fW for a diode having a responsivity

of 0.8 A/W. An optical power of 12.5 µW therefore generates

VLOG = 0.2 V log10(12.5 µW/3.95 fW) = 1.900 V

(12)

In optical applications, the interpretation of VLOG is as an

equivalent optical power; therefore, the slope for calculation

purposes remains 10 mV/dB (for either current or power).

ADL5304ACPZ-R2 数据表(PDF) 17 Page - Analog Devices

ADL5304ACPZ-R2 数据表(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

数据表下载