ꢋ ꢑꢉ ꢀꢁ ꢗꢇ
ꢋ ꢑꢉ ꢀꢁ ꢗꢁ
www.ti.com
SBAS274E − MARCH 2003 − REVISED JUNE 2004
Two approaches can be used for the ground planes: either
a single common plane; or two separate planes, one for the
analog grounds and one for the digital grounds. When
using only one common plane, isolate the flow of current
on pin 57 from pin 1; use breaks on the ground plane to
accomplish this. Pin 57 carries the switching current from
the analog clocking for the modulator and can corrupt the
quiet analog ground on pin 1. When using two planes, it is
recommended that they be tied together right at the PCB.
Do not try to connect the ground planes together after
running separately through edge connectors or cables as
this reduces performance and increases the likelihood of
latchup.
2X MODE
The 2XMODE digital input determines the performance
(16-bit or 14-bit) by setting the oversampling ratio. When
2XMODE = low, the oversampling ratio = 8 for 16-bit
performance. When 2XMODE = high, the oversampling
ratio = 4 for 14-bit performance. Note that when 2XMODE
is high, all 16 bits of DOUT remain active. Decreasing the
oversampling ratio from 8 to 4 doubles the data rate in 2X
mode. For fCLK = 40MHz, the data rate then becomes
10MSPS. In addition, the group delay decreases to 0.9µs
and the settling time becomes 1.3µs or 13 DRDY cycles.
With the reduced oversampling in 2X mode, the noise
increases. Typical SNR performance degrades by 14dB.
THD remains approximately the same. There is an internal
pull-down resistor of 170kΩ on the 2XMODE, however, it
is recommended this pin be forced either high or low. For
more information on the performance of the 2X mode, see
application note Operating the ADS1605 and ADS1606 in
2X Mode: 10MSPS (SLAA180), available for download at
www.ti.com.
In general, keep the resistances used in the driving circuits
for the inputs and reference low to prevent excess thermal
noise from degrading overall performance. Avoid having
the ADS1605/6 digital outputs drive heavy loads. Buffers
on the outputs are recommended unless the ADS1605/6
is connected directly to a DSP or controller situated
nearby. Additionally, make sure the digital inputs are
driven with clean signals as ringing on the inputs can
introduce noise.
LAYOUT ISSUES
The ADS1605/6 uses TI PowerPAD technology. The
PowerPAD is physically connected to the substrate of the
silicon inside the package and must be soldered to the
analog ground plane on the PCB using the exposed metal
pad underneath the package for proper heat dissipation.
Please refer to application report SLMA002, located at
www.ti.com, for more details on the PowerPAD package.
The ADS1605/6 is a very high-speed, high-resolution data
converter. In order to achieve the maximum performance,
careful attention must be given to the printed circuit board
(PCB) layout. Use good high-speed techniques for all
circuitry. Critical capacitors should be placed close to pins
as possible. These include capacitors directly connected
to the analog and reference inputs and the power supplies.
Make sure to also properly bypass all circuitry driving the
inputs and references.
27
BB [ BURR-BROWN CORPORATION ]
