ADRF6518ACPZ-R7 datasheet(24/36 Pages) AD

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ADRF6518

Preliminary Technical Data

Rev. PrA | Page 24 of 36

independent of the analog gain setting. With the digital gain

change, however, VGA output noise changes, because the gain

setting resistors values change. As an example, VGA1 NF

corresponding to a 15 dB gain setting is 14 dB, whereas for 9 dB

gain, the NF is 15.6 dB. When cascaded, the total noise

contributed by the VGAs at the output of the ADRF6518

increases gradually with higher gain. This is apparent in the

noise floor variation at high frequencies at different VGA gain

settings. The exact relationship depends on the programmed

fixed gain of the amplifiers. At lower frequencies within the filter

bandwidth setting, the VGAs translate the filter noise directly to

the output by a factor equal to the gain following the filter.

At low values of VGA gain, the noise at the output is the flat

spectral density contributed by the last VGA. As the gain

increases, more of the filter and first VGA noise appears at

the output. Because the intrinsic filter noise density increases

at lower bandwidth settings, it is more pronounced than it is

at higher bandwidth settings. In either case, the noise density

asymptotically approaches the limit set by the VGAs at the

highest frequencies. For other values of VGA gain and bandwidth

setting, the detailed shape of the noise spectral density changes

according to the relative contributions of the filters and VGAs.

Because the noise spectral density outside the filter bandwidth

is limited by the VGA output noise, it may be necessary to use

an external, fixed frequency, passive filter prior to analog-to-

digital conversion to prevent noise aliasing from degrading the

signal-to-noise ratio. A higher sampling rate, relative to the maxi-

mum required ADRF6518 corner frequency setting, reduces the

order and complexity of this external filter.

DISTORTION CHARACTERISTICS

To maintain low distortion through the cascaded VGAs and

filter of the ADRF6518, consider the distortion limits of each

stage. The first VGA has higher signal handling capability and

bandwidth than VGA2 and VGA3, because it must cope with

out-of-band signals that can be larger than the in-band signals.

In the filter mode, these out-of-band signals are filtered before

reaching VGA2 and VGA3. It is important to understand the

signals presented to the ADRF6518 and to match these signals

with the input and output characteristics of the part. It is useful

to partition the ADRF651 into the front end, composed of

VGA1 and the filter, and the back end, composed of VGA2 and

VGA3 and the output buffers.

VGA1 can handle a maximum analog attenuation setting of

5 V p-p without experiencing appreciable distortion at the

input. In most applications, VGA1 gain should be adjusted such

that the maximum signal presented at the filter inputs (or

VGA2 input in filter bypass mode) is <1.5 V p-p. At this level,

the front end does not limit the distortion performance. The

peak detector output, VPK, can be used as an indicator of the

signal level present at this critical interface. Choose the second

and third VGA gains such that their output level does not

exceed 1 V p-p. The output buffer gain should be set to 3 dB if

the desired output is <1.4 V p-p and 9 dB for a desired output of

>1.4 V p-p.

For these signal level considerations, the out-of-band signal, if

larger than the desired in-band signal, should be addressed. In

filter active mode, such an out-of-band signal only affects the

VGA1 operation, because it is filtered out by the filter and does

not affect the following stages. In this case, a high VGA2 and

VGA3 gain may be needed to raise the small desired signal to a

higher level at the output. In the filter bypass mode, such out-

of-band signals may need to be filtered prior to the ADRF6518.

The overall distortion introduced by the part depends on the

input drive level, including the out-of-band signals, and the

desired output signal level. To achieve best distortion

performance and the desired overall gain, keep in mind the

maximum signal levels indicated previously when selecting

different VGA gains.

To distinguish and quantify the distortion performance of the

input section, two different IP3 specifications are presented.

The first is called in-band IP3 and refers to a two-tone test

where the signals are inside the filter bandwidth. This is exactly

the same figure of merit familiar to communications engineers

in which the third-order intermodulation level, IMD3, is

measured.

To quantify the effect of out-of-band signals, a new out-of-band

(OOB) IIP3 figure of merit is introduced. This test also involves

a two-tone stimulus; however, the two tones are placed out-of-

band so that the lower IMD3 product lands in the middle of the

filter pass band. At the output, only the IMD3 product is visible

because the original two tones are filtered out. To calculate the

OOB IP3 at the input, the IMD3 level is referred to the input by

the overall gain. The OOB IIP3 allows the user to predict the

impact of out-of-band blockers or interferers at an arbitrary

signal level on the in-band performance. The ratio of the desired

input signal level to the input-referred IMD3 at a given blocker

level represents a signal-to-distortion limit imposed by the out-

of-band signals.

MAXIMIZING THE DYNAMIC RANGE

When used in the filter mode, the role of the ADRF6518 is to

increase the level of a variable in-band signal while minimizing

out-of-band signals. Ideally, this is achieved without degrading

the SNR of the incoming signal or introducing distortion to the

incoming signal.

The first goal is to maximize the output signal swing, which can

be defined by the ADC input range or the input signal capacity

of the next analog stage. For the complex waveforms often encoun-

tered in communication systems, the peak-to-average ratio, or

crest factor, must be considered when choosing the peak-to-peak

output. From the chosen output signal and the maximum gain

of the ADRF6518, the minimum input level can be defined.

As the input signal level increases, the VGA3 gain is reduced

from its maximum gain point to maintain the desired fixed

output level. VGA2 and VGA1 can then be adjusted as the input

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