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Data Sheet

ADRF6510

Rev. B | Page 17 of 32

PROGRAMMING THE FILTERS

The 0.5 dB corner frequencies for both filters are programmed

simultaneously through the SPI port. A 5-bit register stores the

codes for corner frequencies of 1 MHz through 30 MHz (see

Table 4). The SPI protocol not only allows frequency codes to

be written to the DATA pin but also allows the stored code to

be read back from the SDO pin.

The latch enable (LE) pin must first go to a Logic 0 for a read or

write cycle to begin. On the next rising edge of the clock (CLK),

a Logic 1 on the DATA pin initiates a write cycle, whereas a

Logic 0 on the DATA pin initiates a read cycle. In a write cycle,

the next five CLK rising edges latch the frequency code, LSB

first. When LE goes high, the write cycle is completed and the

frequency code is presented to the filter. In a read cycle, the next

five CLK falling edges present the stored frequency code, LSB

first. When LE goes high, the read cycle is completed. Detailed

timing diagrams are shown in Figure 2 and Figure 3.

Table 4. Frequency Code vs. Corner Frequency Lookup Table

5-Bit Binary Frequency Code1

Corner Frequency (MHz)

00000

00001

00010

00011

00100

00101

00110

00111

01000

01001

01010

01011

01100

01101

01110

01111

10000

10001

10010

10011

10100

10101

10110

10111

11000

11001

11010

11011

11100

11101

11110

11111

MSB first.

NOISE CHARACTERISTICS

The output noise behavior of the ADRF6510 depends on the gain

and bandwidth settings. Both the filter sections and the VGAs

contribute to the total noise at the output. The filter contributes

a noise spectral density profile that is flat at low frequencies, peaks

near the corner frequency, and then rolls off as the filter poles

roll off the gain. The magnitude of the noise spectral density,

expressed in nV/√Hz, varies inversely with the square root of

the bandwidth setting, resulting in a total integrated noise in

nV that is nearly constant with bandwidth setting.

The X-AMP type VGAs used in the ADRF6510 contribute

a fixed noise spectral density to the output, independent of

the gain setting, of −130 dBV/√Hz, which is equivalent to

316 nV/√Hz. Although the VGA noise contribution to the

output is fixed, the gain of the VGA controls the relative

contribution of the filter noise.

Figure 47 and Figure 48 show the total output noise spectral

density vs. frequency for different bandwidth settings. At low

values of VGA gain, the noise at the output is the flat spectral

density contributed by the VGA because the filter noise is sup-

pressed by the VGA attenuation. As the gain increases, more

of the filter noise appears at the output. Because the filter noise

increases at lower bandwidth settings, it overwhelms the VGA

noise floor. In either case, the noise density asymptotically

approaches the −130 dBV/√Hz limit set by the VGA at the

highest frequencies. For other values of VGA gain and band-

width setting, the detailed shape of the noise spectral density

changes.

–115

–120

–125

–130

–135

FREQUENCY (MHz)

BANDWIDTH = 20MHz

GAIN = 20dB

GAIN = 0dB

GAIN = 40dB

Figure 47. Total Output Noise with a 20 MHz Corner Frequency

for Three Different Gain Settings

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