AD9865BCPZ datasheet(35/49 Pages) AD

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AD9865

Data Sheet

Rev. B | Page 34 of 48

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. Figure 68 shows that the first real pole is im-

plemented within the first CPGA gain stage, and the complex

pole pair is implemented in the second CPGA gain stage.

Capacitor arrays are used to vary the different R-C time con-

stants 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

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 21 shows the SPI registers pertaining to the LPF.

Table 21. SPI Registers for Rx Low-Pass Filter

Address (Hex)

Bit

Description

0x07

(0)

Enable Rx LPF

0x08

(7:0)

Target value

The normalized wideband gain response is shown in Figure 70.

The normalized pass-band gain and group delay responses are

shown in Figure 71. The normalized cutoff frequency, f−3 dB,

results in −3 dB attenuation. Also, 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)

(7)

FREQUENCY

1.0

3.0

–35

–30

–25

–20

–15

–10

–5

2.5

2.0

1.5

0.5

Figure 70. LPF’s Normalized Wideband Gain Response

NORMALIZED FREQUENCY

0.5

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 71. LPF’s Normalized Pass-Band Gain and Group Delay Responses

The −3 dB cut-off frequency, 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 72

shows how the frequency response, 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 72. 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

(8)

Figure 73 compares the measured and calculated f−3 dB using this

formula.

1 Empirically derived for a f−3 dB range of 15 MHz to 35 MHz and fADC of 40 MSPS

to 80 MSPS with an RxPGA = 0 dB.

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