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Category: Hardware

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 kΩ.

100000: IRSET = 1.6 kΩ.

111111: IRSET = 2 kΩ.

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.

## Top Replies

• Thank you for patiently waiting. Regarding xRset (Q2), we've confirmed on bench that the correct xRset for 0b000000 and 0b111111 should be 4kΩ, not 2kΩ. The graph in Fig. 99…

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• Hi  ,

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.

IRSET (Register 0x04 [5:0]) and QRSET (Register 0x07 [5:0]) is designed to operate in two's complement format. This is why register codes 0b000000 and 0b111111 can be approximated to have the same nominal values.

Q2: If IRSET equals 2k then the IOUTFS shouldn't be 32*Vref/2k = 16mA (as per page 45)? Figure 99. looks different.

We will confirm the values in Figure 99 on bench and get back to you.

Q3. Where is the tolerance of xRSET? (I didn't find it on the datasheet.)

The xRSET has a tolerance of 10% allowable error based on our ATE test standards.

Regards,

Marco

• Hello Marco,

Thank you very much for your response.

Q1: I'm not sure that I understand your answer well... if 0b000000 and 0b111111 register values are in two's complement then 0b000000 = 0, 0b111111=-1. The MSB is the sign bit and the others represent the magnitude. The nominal values are different. Is there something that I'm overlooking or not noticing?

Also, the Figure 99 is embarrassing me a bit... the xRSET registers values are in signed format, but the x scale numbers are unsigned. The two's complement values are written as straight decimal?

Regards,

Molnar

• Hello Marco,

Thank you very much for your response.

Q1: I'm not sure that I understand your answer well... if 0b000000 and 0b111111 register values are in two's complement then 0b000000 = 0, 0b111111=-1. The MSB is the sign bit and the others represent the magnitude. The nominal values are different. Is there something that I'm overlooking or not noticing?

Also, the Figure 99 is embarrassing me a bit... the xRSET registers values are in signed format, but the x scale numbers are unsigned. The two's complement values are written as straight decimal?

Regards,

Molnar

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