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AD5781/AD5791 Quick Start Guide

Single, 18-/20-Bit, Voltage Output DACs, SPI Interface

Figure 1. Functional Block Diagram

Features

High relative accuracy (INL): ±0.5 LSB maximum (18-bit AD5781)
1 ppm resolution, 1 ppm INL (20-bit AD5791)
7.5 nV/√Hz output noise spectral density
0.19 LSB long-term linearity error stability (20-bit AD5791)
<0.05 ppm/°C temperature drift
1 μs output voltage settling time
1.4 nV-sec midscale glitch impulse
Operating temperature range: −40°C to +125°C
20-lead TSSOP package
Wide power supply range of up to ±16.5 V
35 MHz Schmitt triggered digital interface
1.8 V compatible digital interface

Pin Configuration

Figure 2. 24-Lead LFCSP Pin Configuration

Table 1. Function Descriptions for Quick Start

Mnemonic	Description
INV	Inverting input connection for external amplifier.
V_OUT	Analog output voltage.
V_REFPS	Positive reference sense voltage input. Connect a voltage in the range of 5 V to V_DD - 2.5 V.
V_REFPF	Positive reference force voltage input. Connect a voltage in the range of 5 V to V_DD - 2.5 V.
V_DD	Positive analog supply connection. Connect a voltage in the range of 7.5 V to 16.5 V. V_DD must be decoupled to AGND.
	Active low reset. Asserting this pin returns the DAC to its power-on status.
	Active low input. Asserting this pin sets the DAC register to a user defined value and updates the DAC output.
	Active low load DAC logic input. This is used to update the DAC register and, consequently, the analog output.
V_CC	Digital supply. Connect a voltage in the range of 2.7 V to 5.5 V. V_CC must be decoupled to DGND.
IOV_CC	Digital interface supply. Voltage range is from 1.71 V to 5.5 V.
SDO	Serial data output.
SDIN	Serial data input.
SCLK	Serial clock input. Data can be transferred at clock rates of up to 35 MHz.
	Active low digital interface synchronization input. This is the frame synchronization signal for the input data.
DGND	Ground reference for digital circuitry.
V_REFNF	Negative reference force voltage input. Connect a voltage in the range of V_SS + 2.5 V to 0 V.
V_REFNS	Negative reference sense voltage input. Connect a voltage in the range of V_SS + 2.5 V to 0 V.
V_SS	Negative analog supply connection. Connect a voltage in the range of -16.5 V to -2.5 V. V_SS must be decoupled to AGND.
AGND	Ground reference for analog circuitry.
R_FB	Feedback connection for external amplifier.

Hardware Control Pins Truth Table

Table 2. Hardware Control Pins Truth Table

/LDAC	/CLR	/RESET	Function
X¹	X¹	0	DAC in reset mode. The device cannot be programmed.
X¹	X¹	⇑²	DAC is turned to its power-on state. All registers are set to their default values.
0	0	1	DAC register loaded with the clearcode register value and output set accordingly.
0	1	1	Output set according to the DAC register value.
1	0	1	DAC register loaded with the clearcode register value and output set accordingly.
⇓³	1	1	Output set according to the DAC register value.
⇓³	0	1	Output remains at the clearcode register value.
⇑²	1	1	Output remains set according to the DAC register value.
⇑²	0	1	Output remains at the clearcode register value.
1	⇓³	1	DAC register loaded with the clearcode register value and output set accordingly.
0	⇓³	1	DAC register loaded with the clearcode register value and output set accordingly.
1	⇑²	1	Output remains at the clearcode register value.
0	⇑²	1	Output set according to the DAC register value.

¹ X is don't care.
² ⇑ is rising edge.
³ ⇓ is falling edge.

Input Shift Register Contents

Figure 3. Input Shift Register Contents (AD5781/AD5791)

Table 3. Register Address Definitions

	Register Address
Read/Write (R/W)	C2	C1	C0	Description
X¹	0	0	0	No operation
0	0	0	1	Write to the DAC register
0	0	1	0	Write to the control register
0	0	1	1	Write to the clearcode register
0	1	0	0	Write to the software control register
1	0	0	1	Read from the DAC register
1	0	1	0	Read from the control register
1	0	1	1	Read from the clearcode register

¹ X = don't care.

Control Register

Figure 4. Control Register (AD5781/AD5791)

Table 4. Control Register Functions

Bit Name	Description
RBUF	Output amplifier configuration control.
	Setting	Function
	0	Internal amplifier powered up.
	1 (default)	Internal amplifier powered down.
OPGND	Output ground clamp control.
	Setting	Function
	0	DAC output clamp to ground removed and DAC placed in normal mode.
	1 (default)	DAC output clamped to ground and DAC placed in tristate mode.
DACTRI	DAC tristate control.
	Setting	Function
	0	DAC in normal operating mode.
	1 (default)	DAC in tristate mode.
BIN/2sC	DAC register coding selection.
	Setting	Function
	0 (default)	DAC register uses twos complement coding.
	1	DAC register uses offset binary coding.
SDODIS	SDO pin enable/disable control.
	Setting	Function
	0 (default)	SDO pin enabled.
	1	SDO pin disabled (tristate).
LIN COMP	Linearity error compensation for varying reference input spans. Note that the reference input span options are 10 V and 20 V for the AD5781. See the AD5781 data sheet for further details.
	Setting	Function
	0000 (default)	Reference input span up to 10 V.
	1001	Reference input span between 10 V and 12 V.
	1010	Reference input span between 12 V and 16 V.
	1011	Reference input span between 16 V and 19 V.
	1100	Reference input span between 19 V and 20 V.
R/	Read/write select bit.
	Setting	Function
	0	AD5781/AD5791 addressed for a write operation.
	1	AD5781/AD5791 addressed for a read operation.

Software Control Register

Figure 5. Software Control Register (AD5781/AD5791)

Table 5. Software Control Register Functions

Bit Name	Description
LDAC¹	Setting this bit to 1 updates the DAC register and consequently the DAC output.
CLR²	Setting this bit to 1 sets the DAC register to a user defined value and updates the DAC output.
RESET	Setting this bit to 1 returns the AD5781/AD5791 to its power-on state.

¹ The LDAC function has no effect when the pin is low. Refer to Table 2 in the Hardware Control Pins Truth Table section for further details.
² The CLR function has no effect when the pin is low. Refer to Table 2 in the Hardware Control Pins Truth Table section for further details.

Transfer Function

V_OUT = (V_REFP - V_REFN) * D/(2^N-1) + V_REFN

where:
V_REFN is the negative voltage applied at the V_REFNx input pin.
V_REFP is the positive voltage applied at the V_REFPx input pin.
D is the 18-bit (AD5781) or 20-bit (AD5791) code programmed to the DAC.
N is the number of bits.

Example 1: Initializing and Writing to the DAC Register

Initializing the DAC

To initialize the part,

Because this initialization is a write to the part, set the R/W bit to a Logic 0.
Keep the default mode for LIN COMP, SDODIS, and RBUF.
To write in binary coding, select BIN/2sC = 1.
Set DACTRI = 0 and OPGND = 0 to remove the DAC output clamp to ground and place the DAC in normal operating mode.

Write the following over the serial interface: 0010 0000 0000 0000 0001 0010 (R/ bit, three register address bits, 20 data bits).

See Table 6 and Figure 6.

Table 6. Bit Settings to Initialize and Write to the Part

Bit(s)	Bit Name	Setting	Description
23	R/	0	AD5781/AD5791 addressed for a write operation
[9:6]	LIN COMP	0000	Linearity error compensation for a reference input span up to 10 V
5	SDODIS	0	The SDO pin enabled for future readings from the part
4	BIN/2sC	1	Offset binary coding
[3:2]	DACTRI, OPGND	00	Remove the DAC output clamp to ground and place the DAC in normal operating mode
1	RBUF	1	The internal amplifier powered down

To write in binary coding, set BIN/2sC = 1.

The default coding is twos complement. The same 24-bit data impacts the values that the user writes to or reads from the part in a different way depending on the coding selected. The user must verify the coding used by writing to the control register or reading back from it.

Figure 6. Initializing the Part

Writing to the DAC Register

To write a midscale code to the DAC register,

Set R/W = 0 to select the write option from the read/write bit.
Set C[2:0] = 001 for the correspondent register address.
Set D[19:0], the data bits, for a midscale code.

The 24-bit data to write over the serial interface is as follows:

18-bit AD5781: 0001 1000 0000 0000 0000 00XX
20-bit AD5791: 0001 1000 0000 0000 0000 0000

where X = don't care.

See Table 7 and Figure 7.

Table 7. Bit Settings to Write to DAC Register

Bit(s)	Bit Name	Setting	Description
23	R/	0	AD5781/AD5791 addressed for a write operation
[22:20]	C2, C1, C0	001	Write to the DAC register
[19:0]	D19 to D0	1000 0000 0000 0000 00XX or 1000 0000 0000 0000 0000	Midscale code

Figure 7. Write to the DAC Register

Example 2: Clearing the DAC to a Defined Value

Writing to the Clearcode Register

To define the value at which the DAC output is set when the pin or CLR bit in the software control register is asserted, write the desired code to the clearcode register.

For a full-scale clear code, write the following over the serial interface:

18-bit AD5781: 0011 1111 1111 1111 1111 11XX
20-bit AD5791: 0011 1111 1111 1111 1111 1111

where X = don't care.

See Figure 9.

Figure 8. Writing Full-Scale Code to the Clearcode Register

Writing to the Software Control Register

Set the CLR bit to a Logic 1 to set the DAC register to a user defined value and update the DAC output.

Write the following over the serial interface: 0100 0000 0000 0000 0000 0010

The user should see the DAC output value change to full-scale code.

Figure 9. Clearing the Part to a User Defined Value

Reading Back From the Clearcode Register

To confirm the clearcode value written to the part, read the data from the clearcode register (full scale for this example).

Write the following over the serial interface:

1011 XXXX XXXX XXXX XXXX XXXX

where X = don't care.

See Figure 10.

Remember that this action is a read function. Therefore, set the R/ bit = 1.

D19 to D0, the data bits, are don't care bits because the intention is to read from the part, and not to write to the part.

Figure 10. Readback from the Clearcode Register

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Table of Contents

AD5781/AD5791 Quick Start Guide

Features

Pin Configuration

Hardware Control Pins Truth Table