ADS1606IPAPR datasheet(27/38 Pages) TI

27 / 38 page

ADS1605

ADS1606

SBAS274H − MARCH 2003 − REVISED MAY 2007

www.ti.com

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 fCLK = 40MHz,

the data rate then becomes 10MSPS. In addition, the

group delay decreases to 0.9

µs and the settling time be-

comes 1.3

µs 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 170k

Ω 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

application

report

SLMA002, located at www.ti.com, for more details on

the PowerPAD package.

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