AD9865-EB datasheet(33/48 Pages) AD

33 / 48 page

AD9865

Rev. A | Page 33 of 48

RECEIVE PATH

The receive path block diagram for the AD9865 (or AD9866) is

shown in Figure 68. The receive signal path consists of a 3-stage

RxPGA, a 3-pole programmable LPF, and a 10-bit (or 12-bit)

ADC. Note that the additional two bits of resolution offered by

the AD9866 result in a 3 dB to 5 dB lower noise floor depending

on the RxPGA gain setting and LPF cutoff frequency. Also working

in conjunction with the receive path is an offset correction circuit.

These blocks are discussed in detail in the following sections.

Note that the power consumption of the RxPGA can be modified

via Register 0x13 as discussed in the Power Control and

Dissipation section.

0 TO 6dB

∆ = 1dB

–6 TO 18dB

∆ = 6dB

–6 TO 24dB

∆ = 6dB

XTAL

RX–

10/12

CONTROL

CLK

SYN.

ADC

80MSPS

CLKOUT_1

CLKOUT_2

OSCIN

RX+

2M CLK

MULTIPLIER

2-POLE

LPF

1-POLE

LPF

SPORT

PGA[5:0]

RXCLK

RXEN/SYNC

ADIO[11:6]/

Tx[5:0]

ADIO[11:6]/

Rx[5:0]

GAIN

MAPPING

LUT

SPGA

AD9865/AD9866

Figure 68. Functional Block Diagram of Rx Path

RX PROGRAMMABLE GAIN AMPLIFIER

The RxPGA has a digitally programmable gain range from

−12 dB to +48 dB with 1 dB resolution via a 6-bit word. Its

purpose is to extend the dynamic range of the Rx path such that

the input of the ADC is presented with a signal that scales

within its fixed 2 V input span. There are multiple ways of

setting the RxPGA’s gain as discussed in the RxPGA Control

section, as well as an alternative 3-bit gain mapping having a

range of −12 dB to +36 dB with 8 dB resolution.

The RxPGA is comprised of two sections: a continuous time

PGA (CPGA) for course gain and a switched capacitor PGA

(SPGA) for fine gain resolution. The CPGA consists of two

cascaded gain stages providing a gain range from −12 dB to

+42 dB with 6 dB resolution. The first stage features a low noise

preamplifier (< 3.0 nV/rtHz), thereby eliminating the need for

an external preamplifier. The SPGA provides a gain range from

0 dB to 6 dB with 1 dB resolution. A look-up table (LUT) is

used to select the appropriate gain setting for each stage.

The nominal differential input impedance of the RxPGA input

appearing at the device RX+ and RX− input pins is 400 Ω//4 pF

(±20%) and remains relatively independent of gain setting. The

PGA input is self-biased at a 1.3 V common-mode level allowing

maximum input voltage swings of ±1.5 V at RX+ and RX−. AC

coupling the input signal to this stage via coupling capacitors

(0.1 µF) is recommended to ensure that any external dc offset

does not get amplified with high RxPGA gain settings,

potentially exceeding the ADC input range.

To limit the RxPGA’s self-induced input offset, an offset

cancellation loop is included. This cancellation loop is auto-

matically performed upon power-up and can also be initiated

via SPI. During calibration, the RxPGA’s first stage is internally

shorted, and each gain stage set to a high gain setting. A digital

servo loop slaves a calibration DAC, which forces the Rx input

offset to be within ±32 LSB for this particular high gain setting.

Although the offset varies for other gain settings, the offset is

typically limited to ±5% of the ADC’s 2 V input span. Note that

the offset cancellation circuitry is intended to reduce the voltage

offset attributed to only the RxPGA’s input stage, not any dc

offsets attributed to an external source.

The gain of the RxPGA should be set to minimize clipping of

the ADC while utilizing most of its dynamic range. The maxi-

mum peak-to-peak differential voltage that does not result in

clipping of the ADC is shown in Figure 69. While the graph

suggests that maximum input signal for a gain setting of −12 dB

is 8.0 V p-p, the maximum input voltage into the PGA should

be limited to less than 6 V p-p to prevent turning on ESD protec-

tion diodes. For applications having higher maximum input

signals, consider adding an external resistive attenuator network.

While the input sensitivity of the Rx path is degraded by the

amount of attenuation on a dB-to-dB basis, the low noise

characteristics of the RxPGA provide some design margin such

that the external line noise remains the dominant source.

GAIN (dB)

–12

–6

182430

3642

8.0000

4.0000

2.0000

1.0000

0.5000

0.2500

0.1250

0.0625

0.0312

0.0156

0.0100

Figure 69. Maximum Peak-to-Peak Input vs. RxPGA Gain Setting

that Does Not Result in ADC Clipping

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