ADL5306ACP-R2 datasheet(10/16 Pages) AD

10 / 16 page

ADL5306

Rev. 0 | Page 10 of 16

It is apparent that this output should be zero for IPD = IREF, and

would need to swing negative for smaller values of input

current. To avoid this, IREF would need to be as small as the

smallest value of IPD. In the ADL5306, an internal offset voltage

is added to VLOG to shift it upward by 0.8 V. This moves the

intercept to the left by four decades, from 10 µA to 1 nA:

ILOG = IY log10(IPD / IINTC)

(4)

where IINTC is the operational / value of the intercept current.

Since values of IPD < IINTC result in a negative VLOG, a negative

supply of sufficient value is required to accommodate this

situation (discussed later).

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 the 14.2 kΩ resistor to the

internal 2.5 V reference. At the VLOG pin, the output current

ILOG generates a voltage of

VLOG = ILOG × 4.55 kΩ

= 44 µA × 4.55 kΩ × log10 (IPD / IREF)

(5)

= VY log10 (IPD / IREF)

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

resistive loading on VLOG will lower this slope and will result

in an overall scaling uncertainty due to the variability of the on-

chip resistors. Consequently, 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 more negative, the input

pins INPT and IREF may be positioned at ground level simply

by grounding VSUM.

MANAGING INTERCEPT AND SLOPE

As previously noted, the internally generated 2.5 V bias

combines with the on-chip resistors to introduce an accurate

offset voltage of 0.8 V at the VLOG pin, equivalent to four

decades. This results 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 10 nA when using the

recommended value of IREF = 100 µA.

The slope can be reduced by attaching a resistor to the VLOG

pin. This is strongly discouraged because the on-chip resistors

will 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 necessary, it should be done at the low impedance output

of the buffer, which is provided to avoid such miscalibration

and allow higher slopes to be used.

The ADL5306 buffer is essentially an uncommitted op amp

with rail-to-rail output swing, good load driving capabilities,

and a unity-gain bandwidth of >20 MHz. In addition to

allowing the introduction of gain using standard feedback

networks, 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. For more details, see

the AD8304 Data Sheet.

RESPONSE TIME AND NOISE CONSIDERATIONS

The response time and output noise of the ADL5306 are

fundamentally a function of the signal current IPD. For small

currents, the bandwidth is proportional to IPD. The output’s low

frequency voltage-noise spectral density is a function of IPD, and

increases for small values of IREF. For details of noise and

bandwidth performance of translinear log amps, see the

AD8304 Data Sheet.

ADL5306ACP-R2 数据表(PDF) 10 Page - Analog Devices

ADL5306ACP-R2 数据表(HTML) 10 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

数据表下载