REV. 0

AD9875

–16–

D/A CONVERTER

The AD9875 DAC provides differential output current on the

Tx+ and Tx– pins. The value of the output currents are compli-

current of the DAC. For example, when the current from Tx+ is

at full-scale, the current from Tx– is zero. The two currents will

typically drive a resistive load which will convert the output

currents to a voltage. The Tx+ and Tx– output currents are

inherently ground seeking and should each be connected to

The full-scale output current of the DAC is set by the value of

the resistor placed from the FSADJ pin to AGND. The relation-

is governed by the following equation:

The full-scale current can be set from 2 mA to 20 mA. Gener-

ally, there is a trade-off between DAC performance and power

consumption. The best DAC performance will be realized at an

overall current consumption of the device.

The single-ended voltage output appearing at the Tx+ and Tx–

nodes are:

× R

L

× R

L

exceed the maximum output compliance range of 1.5 V to pre-

vent signal compression. To maintain optimum distortion and

should not exceed 0.5 V.

The single ended full-scale voltage at either output node will be:

× R

L

× R

L

and

× R

L

For optimum performance, a differential output interface is

recommended since any common-mode noise or distortion can

be supressed.

It should be noted that the differential output impedance of the

DAC is 2

× R

resistors will load down the output voltage accordingly.

RECEIVE PATH DESCRIPTION

The receive path consists of a two-stage PGA, a continuous time,

4-pole LPF, an ADC, a digital HPF and a digital data multiplexer.

Also working in conjunction with the receive path is an offset

correction circuit and a digital phase lock loop. Each of these

blocks will be discussed in detail in the following sections.

PROGRAMMABLE GAIN AMPLIFIER

The PGA has a programmable gain range from –6 dB to +36 dB

if the narrower (approximately 12 MHz) LPF bandwidth is

selected, or if the LPF is bypassed. If the wider (approximately

26 MHz) LPF bandwidth is selected, the gain range is –6 dB to

+30 dB. The PGA is comprised of two sections, a Continuous

Time PGA (CPGA) and a Switched Capacitor PGA (SPGA).

The CPGA has possible gain settings of –6, 0, 6, 12, 18, and 24.

The SPGA has possible gain settings of 0, 2, 4, 6, 8, 10, and 12 dB.

Table I shows how the gain is distributed for each programmed

gain setting.

The CPGA input appears at the device Rx+ and Rx– input pins.

The input impedance of this stage is nominally 270

Ω differen-

tial and is not gain dependent. It is best to ac-couple the input

signal to this stage and let the inputs self bias. This will lower the

offset voltage of the input signal, which is important at higher

gains, as any offset will lower the output compliance range of the

CPGA output. When the inputs are driven by direct coupling, the

dc level should be AVDD/2. However, this could lead to larger dc

offsets and consequently reduce the dynamic range of the Rx path.

LOW-PASS FILTER

The Low-Pass Filter (LPF) is a programmable, multistage,

fourth order low-pass filter comprised of two real poles and a

complex pole pair. The first real pole is implemented within the

CPGA. The second filter stage implements a complex pair of

poles. The last real pole is implemented in a buffer stage that

drives the SPGA.

There are two passband settings for the LPF. Within each pass-

band the filters are tunable over about a

30% frequency range.

The formula for the cutoff frequency is:

× 64/(64 + Target)

× 158/(64 + Target)

Where Target is the decimal value programmed as the tuning

target in Register 5.

This filter may also be bypassed by setting Bit 0 of Register 4.

In this case, the bandwidth of the Rx path will decrease with

increasing gain and be approximately 50 MHz at the highest

gain settings.

ADC

The AD9875’s analog-to-digital converter implements a pipelined

multistage architecture to achieve high sample rates while con-

suming low power. The ADC distributes the conversion over

several smaller A/D subblocks, refining the conversion with

progressively higher accuracy as it passes the results from stage

to stage. As a consequence of the distributed conversion, ADCs

require a small fraction of the 2

tional n-bit flash-type A/D. A sample-and-hold function within

each of the stages permits the first stage to operate on a new

input sample while the remaining stages operate on preceding

samples. Each stage of the pipeline, excluding the last, consists

of a low resolution flash A/D connected to a switched capacitor

DAC and interstage residue amplifier (MDAC). The residue

amplifier amplifies the difference between the reconstructed

DAC output and the flash input for the next stage in the pipe-

line. One bit of redundancy is used in each one of the stages to

facilitate digital correction of flash errors. The last stage simply

consists of a flash A/D.