AD9117 coarse gain setting

Question

Hello, 
 we are playing the gain control of an AD9117 DAC. 
 Config: VDD is 3.3V, Vref is 1V, IRSET is enabled and we have IV converter with 49.9 ohm resistors at the DAC outputs. CMPLx grounded, RLxP/N open. 
 Stated in datasheet rev. E p.38: 
 Changes the value of the on-chip IRSET resistor; this scales the full-scale current of the DAC in ~0.25 dB steps twos complement (nonlinear), see Figure 99. 
 000000 (default): IRSET = 2 kΩ. 
 011111: IRSET = 8 k&Omega;. 
 100000: IRSET = 1.6 k&Omega;. 
 111111: IRSET = 2 k&Omega;. 
 Q1: How is it possible to have same IRSET values (2k) with different register codes (000000, 111111)? They seem one step far, so the resistance must be differ. 
 Q2: If IRSET equals 2k then the IOUTFS shouldn't be 32*Vref/2k = 16mA (as per page 45)? Figure 99. looks different. 
 Q3. Where is the tolerance of xRSET? (I didn't find it on the datasheet.) 
 
 Thank you.

JMMina · Accepted Answer

Hello molnar , 
 Thank you for patiently waiting. Regarding xRset (Q2), we've confirmed on bench that the correct xRset for 0b000000 and 0b111111 should be 4k &Omega; , not 2k &Omega; . The graph in Fig. 99 is correct. The values listed in page 38 Table 14 will be lined up for datasheet revision. 
 For Fig. 99, although there is a 1-bit difference between register codes 0b000000 and 0b111111 in two's complement format, their corresponding analog resistance values (xRset) can be approximated to be equal at 4k &Omega; based on the resolution of the xRset register (IRSET 0x04 [5:0] and QRSET 0x07 [5:0]). The x-scale numbers in the graph are not the equivalent (unsigned) decimal values of the xRset values in two's complement format. Rather, the numbers are in proportion to negative full-scale and positive full-scale values. 
 Regards, 
 Marco

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AD9117 coarse gain setting

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