Datasheet

14-Bit
ADC
14b
DigitalFunctions
(Gain,OffsetCorrection,TestPatterns)
Output
Interface
DDRLVDS
orCMOS
14b
DISLOWLATENCYPin
ADS4122, ADS4125
ADS4142, ADS4145
www.ti.com SBAS520A –FEBRUARY 2011– REVISED MARCH 2011
DIGITAL FUNCTIONS AND LOW LATENCY MODE
The device has several useful digital functions such as test patterns, gain, and offset correction. All of these
functions require extra clock cycles for operation and increase the overall latency and power of the device.
Alternately, the device has a low-latency mode in which the raw ADC output is routed to the output data pins with
a latency of 10 clock cycles. In this mode, the digital functions are bypassed. Figure 110 shows more details of
the processing after the ADC.
The device is in low-latency mode after reset. In order to use any of the digital functions, the low-latency mode
must first be disabled by setting the DIS LOW LATENCY register bit to '1'. After this, the respective register bits
must be programmed as described in the following sections and in the Serial Register Map section.
Figure 110. Digital Processing Block Diagram
GAIN FOR SFDR/SNR TRADE-OFF
The ADS412x/4x include gain settings that can be used to improve SFDR performance. The gain is
programmable from 0dB to 6dB (in 0.5dB steps) using the GAIN register bits. For each gain setting, the analog
input full-scale range scales proportionally, as shown in Table 9.
The SFDR improvement is achieved at the expense of SNR; for each gain setting, the SNR degrades
approximately between 0.5dB and 1dB. The SNR degradation is reduced at high input frequencies. As a result,
the gain is very useful at high input frequencies because the SFDR improvement is significant with marginal
degradation in SNR. Therefore, the gain can be used to trade-off between SFDR and SNR.
After a reset, the device is in low-latency mode and gain function is disabled. To use gain:
• First, disable the low-latency mode (DIS LOW LATENCY = 1).
• This setting enables the gain and puts the device in a 0dB gain mode.
• For other gain settings, program the GAIN bits.
Table 9. Full-Scale Range Across Gains
GAIN (dB) TYPE FULL-SCALE (V
PP
)
0 Default after reset 2
1 Programmable 1.78
2 Programmable 1.59
3 Programmable 1.42
4 Programmable 1.26
5 Programmable 1.12
6 Programmable 1.00
Copyright © 2011, Texas Instruments Incorporated Submit Documentation Feedback 63
Product Folder Link(s): ADS4122 ADS4125 ADS4142 ADS4145