Datasheet

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SBAS274H − MARCH 2003 − REVISED MAY 2007
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27
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 over-
sampling ratio = 4 for 14-bit performance. Note that
when 2XMODE is high, all 16 bits of DOUT remain ac-
tive. Decreasing the oversampling ratio from 8 to 4
doubles the data rate in 2X mode. For f
CLK
= 40MHz,
the data rate then becomes 10MSPS. In addition, the
group delay decreases to 0.9µs and the settling time be-
comes 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 rec-
ommended 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.
LAYOUT ISSUES
The ADS1605/6 is a very high-speed, high-resolution
data converter. In order to achieve the maximum perfor-
mance, careful attention must be given to the printed
circuit board (PCB) layout. Use good high-speed tech-
niques for all circuitry. Critical capacitors should be
placed close to pins as possible. These include capaci-
tors directly connected to the analog and reference in-
puts and the power supplies. Make sure to also properly
bypass all circuitry driving the inputs and references.
Two approaches can be used for the ground planes: ei-
ther 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 mod-
ulator 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 per-
formance and increases the likelihood of latchup.
In general, keep the resistances used in the driving cir-
cuits 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 con-
troller situated nearby. Additionally, make sure the digi-
tal inputs are driven with clean signals as ringing on the
inputs can introduce noise.
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.