AD9869BCPZ datasheet(26/37 Pages) AD

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AD9869

Rev. A | Page 25 of 36

LOW-PASS FILTER

The low-pass filter (LPF) provides a third-order response with a

cutoff frequency that is typically programmable over a 15 MHz

to 35 MHz span. The first real pole is implemented within the

first CPGA gain stage (see Figure 24), and the complex pole pair

is implemented in the second CPGA gain stage. Capacitor

arrays are used to vary the different RC time constants within

these two stages in a manner that changes the cutoff frequency

while preserving the normalized frequency response. Because

absolute resistor and capacitor values are process-dependent, a

calibration routine lasting less than 100 μs automatically occurs

each time the target cutoff frequency register (Register 0x08) is

updated, ensuring a repeatable cutoff frequency from device to

device.

Although the default setting specifies that the LPF be active, it

can also be bypassed providing a nominal f−3 dB of 55 MHz.

Table 19 shows the SPI registers pertaining to the LPF.

Table 19. SPI Registers for Rx Low-Pass Filter

Address (Hex)

Bit

Description

0x07

Enable Rx LPF.

0x08

7:0

Target value.

The normalized wideband gain response is shown in Figure 26.

The normalized pass-band gain and group delay responses are

shown in Figure 27. The −3 dB cutoff frequency, f−3 dB, results in

−3 dB attenuation. In addition, the actual group delay time

(GDT) response can be calculated given a programmed cutoff

frequency using the following equation:

Actual GDT = Normalized GDT/(2.45 × f−3dB)

(4)

FREQUENCY

1.0

3.0

–35

–30

–25

–20

–15

–10

–5

2.5

2.0

1.5

0.5

Figure 26. LPF Normalized Wideband Gain Response

NORMALIZED FREQUENCY

00.5

1.0

0.9

0.3

0.4

0.8

0.2

0.1

1.30

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

0.25

–3.00

–2.75

–2.50

–2.25

–2.00

–1.75

–1.50

–1.25

–1.00

–0.75

–0.50

–0.25

0.6

0.7

NORMALIZED GROUP DELAY

NORMALIZED GAIN RESPONSE

Figure 27. LPF Normalized Pass-Band Gain and Group Delay Responses

The f−3 dB is programmable by writing an 8-bit word, referred to

as the target, to Register 0x08. The cutoff frequency is a function

of the ADC sample rate, fADC, and to a lesser extent, the RxPGA

gain setting (in dB). Figure 28 shows how f−3 dB varies as a

function of the RxPGA gain setting.

INPUT FREQUENCY (MHz)

010

–18

–12

–6

–15

–9

–3

–6dB GAIN

0dB GAIN

+6dB GAIN

+18dB GAIN

+30dB GAIN

+42dB GAIN

Figure 28. Effects of RxPGA Gain on LPF Frequency Response

(f−3 dB = 32 MHz @ 0 dB and fADC = 80 MSPS)

The following formula1 can be used to estimate f−3 dB for a

RxPGA gain setting of 0 dB:

f−3dB_0dB = (128/target) × (fADC/80) ×(fADC/30 + 23.83) f

(5)

Figure 29 compares the measured and calculated f−3 dB using this

formula.

1 Empirically derived for an f−3 dB range of 15 MHz to 35 MHz and an fADC of

40 MSPS to 80 MSPS with an RxPGA = 0 dB.

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