LM98640QML-SP Datasheet, PDF (29/54 Pages) Texas Instruments – Dual Channel, 14-Bit, 40 MSPS Analog Front End with LVDS Output

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LM98640QML-SP
SNAS461D – MAY 2010 – REVISED SEPTEMBER 2015
Feature Description (continued)
7.3.4 Programmable Analog Offset Correction
The LM98640QML provides two analog DACs per channel to provide flexibility in offset control. Each channel
has a Coarse DAC and Fine DAC which have +/-8bit resolutions (8bit + Sign). The two DACs can be used
independently or as Coarse/Fine configuration. The Coarse DAC provides a sufficient range with moderate step
sizes, while the Fine DAC is for designs which need a finely tuned offset. The correction voltage is applied to the
"Video" level for both Sample & Hold and CDS input modes. Because of the DACs location in the signal path, the
correction range lowers as CDS gain increases, and the output referred correction steps and ranges increase
with PGA gain. Table 1 provides the range and step size of each DAC.
Coarse DAC
Fine DAC
Table 1. Analog Offset DAC Specifications
CDS/SH GAIN
RANGE ⁽¹⁾
STEP SIZE⁽¹⁾
1x
+/- 250 mV
1 mV
2x
+/- 125 mV
500 µV
1x
+/- 5 mV
20 µV
2x
+/- 2.5 mV
10 µV
OUTPUT ADC CODES
+/-2048
+/-2048
+/-41
+/-41
(1) Referred to Input
To use the Offset Correction DACs, the Coarse DAC and Fine DAC must be enabled using bits[6:5] of the Main
Configuration Register (0x00). Then the desired correction value should be entered into the CDAC or FDAC
register of the appropriate channel. The Offset Correction DACs use a signed binary format which is summarized
in Table 2.
Table 2. Analog Offset Correction DAC Format
CDAC / FDAC INPUT VALUE
1 1111 1111
1 0111 1111
0 1111 1111
0 0111 1111
0 0000 0000
CDAC CORRECTION (CODES)
+2048
+1024
0
-1024
-2048
FDAC CORRECTION (CODES)
+41
+20
0
-20
-41
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