AD9716-EBZ datasheet(45/48 Pages) AD

45 / 48 page

AD9714/AD9715/AD9716/AD9717

Rev. 0 | Page 45 of 48

Two registers are assigned to each DAC with 10 bits for the

actual DAC current to be generated, a 3-bit offset (and gain)

adjustment, a 2-bit current range adjustment, and an enable/

disable bit. Setting the QAUXOFS and IAUXOFS bits to all 1s

disables the respective op amp and routes the DAC current

directly to their respective FSADJI/ AUXI or FSADJQ/AUXQ

pins. This is especially useful where the loads to be driven are

beyond the limited capability of the on-chip amplifier. The

DAC output will open circuit when not enabled (QAUXEN

or IAUXEN = 0).

DAC-TO-MODULATOR INTERFACING

The auxiliary DACs can be used for local oscillator (LO) cancella-

tion when the DAC output is followed by a quadrature modulator.

This LO feedthrough is caused by the input referred dc offset

voltage of the quadrature modulator (and the DAC output offset

voltage mismatch) and can degrade system performance. Typical

DAC-to-quadrature modulator interfaces are shown in Figure 94

and Figure 95. Often, the input common-mode voltage for the

modulator is much higher than the output compliance range of

the DAC, so that ac coupling or a dc level shift is necessary. If the

required common-mode input voltage on the quadrature modu-

lator matches that of the DAC, the dc blocking capacitors in

Figure 94 can be removed. A low-pass or band-pass passive filter

is recommended when spurious signals from the DAC (distortion

and DAC images) at the quadrature modulator inputs can affect

the system performance. Placing the filter at the location shown

in Figure 94 and Figure 95 allows easy design of the filter, because

the source and load impedances can easily be designed close to

500 Ω for a 2 mA full-scale output.

AD9714/AD9715/

AD9716/AD9717

AUXDAC1

AD9714/AD9715/

AD9716/AD9717

I DAC

100kΩ

OPTIONAL

PASSIVE

FILTERING

MODULATOR V+

QUADRATURE

MODULATOR

I INPUTS

AD9714/AD9715/

AD9716/AD9717

AUX2DAC

AD9714/AD9715/

AD9716/AD9717

Q DAC

100kΩ

OPTIONAL

PASSIVE

FILTERING

MODULATOR V+

QUADRATURE

MODULATOR

Q INPUTS

Figure 94. Typical Use of Auxiliary DACs and On-Chip Resistors for Direct

Coupling to Quadrature Modulators

AD9714/AD9715/

AD9716/AD9717

AUXDAC

AD9714/AD9715/

AD9716/AD9717

I OR Q DAC

100kΩ

1kΩ

OPTIONAL

PASSIVE

FILTERING

ADL537x FAMILY

I OR Q INPUTS

Figure 95. Typical Use of Auxiliary DACs When DC Coupling to Quadrature

Modulator ADL537x Family

CORRECTING FOR NONIDEAL PERFORMANCE OF

QUADRATURE MODULATORS ON THE IF-TO-RF

CONVERSION

Analog quadrature modulators make it very easy to realize

single sideband radios. However, there are several nonideal

aspects of quadrature modulator performance. Among these

analog degradations are gain mismatch and LO feedthrough.

Gain Mismatch

The gain in the real and imaginary signal paths of the quad-

rature modulator may not be matched perfectly. This leads

to less than optimal image rejection because the cancellation of

the negative frequency image is less than perfect.

LO Feedthrough

The quadrature modulator has a finite dc referred offset, as well

as coupling from its LO port to the signal inputs. These can lead

to a significant spectral spur at the frequency of the Quadrature

Modulator LO.

The AD9714/AD9715/AD9716/AD9717 have the capability

to correct for both of these analog degradations. However,

understand that these degradations drift over temperature;

therefore, if close to optimal single sideband performance

is desired, a scheme for sensing these degradations over

temperature and correcting them may be necessary.

I/Q CHANNEL GAIN MATCHING

Fine gain matching is achieved by adjusting the values in the

DAC fine gain adjustment registers. For the I DAC, these values

are in the I DAC gain register (Register 0x03). For the Q DAC,

these values are in the Q DAC gain register (Register 0x06). These

are 6-bit values that cover ±2% of full scale. To perform gain

compensation starting from the default values of zero, raise the

value of one of these registers a few steps until it can be deter-

mined if the amplitude of the unwanted image is increased or

decreased. If the unwanted image increased in amplitude, remove

the step and try the same adjustment on the other DAC control

improved further. If the fine gain adjustment range is not sufficient

to find a null (that is, the register goes full scale with no null

apparent) adjust the course gain settings of the two DACs

accordingly and try again. Variations on this simple method

are possible.

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