AD6688 datasheet(32/138 Pages) AD

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AD6688

Data Sheet

Rev. 0 | Page 32 of 138

Clock Jitter Considerations

High speed, high resolution ADCs are sensitive to the quality

of the clock input. The degradation in SNR at a given input

frequency (fA) due only to aperture jitter (tJ) can be calculated by

SNRJITTER = −20 × log10 (2 × π × fA × tJ)

In this equation, the rms aperture jitter represents the root

mean square of all jitter sources, including the clock input,

analog input signal, and ADC aperture jitter specifications.

Treat the clock input as an analog signal in cases where aperture

jitter can affect the dynamic range of the AD6688. Separate

power supplies for clock drivers from the ADC output driver

supplies to avoid modulating the clock signal with digital noise.

If the clock is generated from another type of source (by gating,

dividing, or other methods), retime the clock by the original clock

at the last step. Refer to the AN-501 Application Note and the

AN-756 Application Note for more in-depth information about

jitter performance as it relates to ADCs.

The SNR can be estimated by using the following equation:













−











 −











 −

10log

(dBFS)

JITTER

ADC

SNR

POWER-DOWN/STANDBY MODE

The AD6688 has a PDWN/STBY pin that can configure the device

in power-down or standby mode. The default operation is PDWN.

The PDWN/STBY pin is a logic high pin. When in power-down

mode, the JESD204B link is disrupted. The power-down option

can also be set via Register 0x003F and Register 0x0040.

In standby mode, the JESD204B link is not disrupted and

transmits zeros for all converter samples. This can be changed

using Register 0x0571, Bit 7 to select /K/ characters.

TEMPERATURE DIODE

The AD6688 contains diode-based temperature sensors. The

diodes output voltages commensurate to the temperature of the

silicon. There are multiple diodes on the die; however, the

results established using the temperature diode at the central

location of the die can be regarded as representative of the

entire die. However, in applications where only one channel is

used (the other being in the power-down state), it is recommended

to read the temperature diode corresponding to the channel

that is on. Figure 59 shows the locations of the diodes inside the

AD6688 whose voltages can be output to the VREF pin. In each

location, there is a pair of diodes, one of which is 20× the size of

the other. It is recommended to use both diodes in a location to

obtain an accurate estimate of the die temperature. For more

information, see the AN-1432 Application Note, Practical Thermal

Modeling and Measurements in High Power ICs.

ADC

DIGITAL

JESD204B DRIVER

ADC

TEMPERATURE DIODE

LOCATIONS

CHANNEL A, CENTRAL,

CHANNEL B

VREF

Figure 59. Temperature Diode Locations in the Die

The temperature diode voltages can be exported to the VREF

pin using the SPI. Use Register 0x18E6 to enable or disable the

diodes. It is important to note that other voltages can be

exported to the VREF pin at the same time, which can result in

undefined behavior. To ensure a proper readout, switch off all

other voltage exporting circuits as detailed in this section.

Figure 60 shows the block diagram representation of the

controls that are required to enable the diode voltage readout.

VREF

CONTROL

SPI REGISTER

(0x18A6)

TEMPERATURE DIODE

LOCATION SELECT

SPI REGISTER (0x18E6)

CHANNEL A

CENTRAL

CHANNEL B

Figure 60. Register Controls to Output Temperature Diode Voltage on VREF Pin

The SPI writes required to export the central temperature diode

are as follows (see the Memory Map Register Details for more

information):

1. Set Register 0x0008 to 0x03 to select both channels.

2. Set Register 0x18E3 to 0x00 to turn off the VCM export.

3. Set Register 0x18A6 to 0x00 to turn off the voltage

reference export.

4. Set Register 0x18E6 to 0x01 to turn on voltage export of

the central 1× temperature diode. The typical voltage

response of the temperature diode is shown in Figure 61.

Even though this voltage does represent the die

temperature, it is recommended to take measurements

from a pair of diodes for improved accuracy. The following

step explains how to enable the 20× diode.

5. Set Register 0x18E6 to 0x02 to turn on the second central

temperature diode of the pair, which is 20× the size of the

first. For the method utilizing two diodes simultaneously

giving a more accurate result, see the AN-1432 Application

Note, Practical Thermal Modeling and Measurements in

High Power ICs.

The relationship between the measured ΔV and the junction

temperature in degrees Celsius is shown in Figure 62.

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