Datasheet
Table Of Contents
- FEATURES
- APPLICATIONS
- DESCRIPTION
- ABSOLUTE MAXIMUM RATINGS
- RECOMMENDED OPERATING CONDITIONS
- ELECTRICAL CHARACTERISTICS
- ELECTRICAL CHARACTERISTICS
- DIGITAL CHARACTERISTICS
- TIMING SPECIFICATIONS
- SERIAL INTERFACE TIMING CHARACTERISTICS
- RESET TIMING
- TYPICAL CHARACTERISTICS
- APPLICATION INFORMATION
- THEORY OF OPERATION
- ANALOG INPUT
- INPUT COMMON MODE
- REFERENCE
- COARSE GAIN AND PROGRAMMABLE FINE GAIN
- CLOCK INPUT
- CLOCK BUFFER GAIN
- POWER DOWN MODES
- POWER SUPPLY SEQUENCING
- DIGITAL OUTPUT INTERFACE
- OUTPUT BIT ORDER
- MSB/LSB FIRST
- OUTPUT DATA FORMATS
- LVDS CURRENT CONTROL
- LVDS INTERNAL TERMINATION
- CAPTURE TEST PATTERNS
- OUTPUT TIMINGS AT LOWER SAMPLING FREQUENCIES
- DEFINITION OF SPECIFICATIONS
S0165-04
VCM
REFM
REFP
INTREF
INTREF
EXTREF
4kW
1kW
Internal
Reference
ADS6xxx
Internal Reference
External Reference
Full−scale differential input pp + (Voltage forced on VCM) 1.33
(2)
COARSE GAIN AND PROGRAMMABLE FINE GAIN
ADS6425
SLWS197B – MARCH 2007 – REVISED JUNE 2009 .........................................................................................................................................................
www.ti.com
Figure 33. Reference Section
When the device is in internal reference mode, the REFP and REFM voltages are generated internally.
Common-mode voltage (1.5 V nominal) is output on VCM pin, which can be used to externally bias the analog
input pins.
When the device is in external reference mode, the VCM acts as a reference input pin. The voltage forced on the
VCM pin is buffered and gained by 1.33 internally, generating the REFP and REFM voltages. The differential
input voltage corresponding to full-scale is given by Equation 2 .
In this mode, the range of voltage applied on VCM pin should be 1.45 to 1.55V. The 1.5-V common-mode
voltage to bias the input pins has to be generated externally.
The ADS6425 includes gain settings that can be used to get improved SFDR performance (compared to 0 dB
gain mode). The gain settings are 3.5 dB coarse gain and programmable fine gain from 0 dB to 6 dB. For each
gain setting, the analog input full-scale range scales proportionally, as shown in Table 19 .
The coarse gain is a fixed setting of 3.5 dB and is designed to improve SFDR with little degradation in SNR (as
seen in Figure 13 andFigure 14 ). The fine gain is programmable in 1 dB steps from 0 to 6 dB. With the fine gain
also, SFDR improvement is achieved, but at the expense of SNR (there will be about 1dB SNR degradation for
every 1dB of fine gain).
So, the fine gain can be used to trade-off between SFDR and SNR. The coarse gain makes it possible to get
best SFDR but without losing SNR significantly. At high input frequencies, the gains are especially useful as the
SFDR improvement is significant with marginal degradation in SINAD.
The gains can be programmed using the register bits < COARSE GAIN> (Table 16 ) and < FINE GAIN>
(Table 15 ). Note that the default gain after reset is 0 dB.
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Product Folder Link(s): ADS6425