AD7845BQ Datasheet AD7845JRZ, AD7845KRZ, AD7845BRZ | P10-P12

AD7845

REV. B –9–

Figure 17. Programmable Current Sink

4–20 mA CURRENT LOOP

The AD7845 provides an excellent way of making a 4-20 mA

current loop circuit. This is basically a variation of the circuits

in Figures 16 and 17 and is shown in Figure 18. The application

resistor R

(Value 4R) produces the effective 4 mA offset.

= I

+ I

Since I

> I

= –

156

2.5

× R

2.5

DAC

× D × R













156

and since R

DAC

2.5

+ D ×2.5













1000

156

= [4 + (16 × D)]mA, where D goes from 0 to 1 with

Digital Code

When D = 0 (Code of all 0s):

= 4 mA

When D = 1 (Code of all 1s):

= 20 mA

The above circuit succeeds in significantly reducing the circuit

component count. Both the on-chip output amplifier and the

application resistor R

contribute to this.

Figure 18. 4–20 mA Current Loop

APPLICATION HINTS

General Ground Management: AC or transient voltages

between AGND and DGND can cause noise injection into the

analog output. The simplest method of ensuring that voltages at

AGND and DGND are equal is to tie AGND and DGND

together at the AD7845. In more complex systems where the

AGND and DGND intertie is on the backplane, it is recom-

mended that two diodes be connected in inverse parallel be-

tween the AD7845 AGND and DGND pins (IN914 or

equivalent).

Digital Glitches: When a new digital word is written into the

DAC, it results in a change of voltage applied to some of the

DAC switch gates. This voltage change is coupled across the

switch stray capacitance and appears as an impulse on the cur-

rent output bus of the DAC. In the AD7845, impulses on this

bus are converted to a voltage by R

and the output amplifier.

The output voltage glitch energy is specified as the area of the

resulting spike in nV-seconds. It is measured with V

REF

con-

nected to analog ground and for a zero to full-scale input code

transition. Since microprocessor based systems generally have

noisy grounds which couple into the power supplies, the

AD7845 V

and V

terminals should be decoupled to signal

ground.

Temperature Coefficients: The gain temperature coefficient

of the AD7845 has a maximum value of 5 ppm/°C. This corre-

sponds to worst case gain shift of 2 LSBs over a 100°C tem-

perature range. When trim resistors R1 and R2 in Figure 13

are used to adjust full-scale range, the temperature coefficient

of R1 and R2 must be taken into account. The offset tempera-

ture coefficient is 5 ppm of FSR/°C maximum. This corre-

sponds to a worst case offset shift of 2 LSBs over a 100°C

temperature range.

The reader is referred to Analog Devices Application Note

“Gain Error and Gain Temperature Coefficient of CMOS Mul-

tiplying DACs,” Publication Number E630C-5-3/86.

AD7845

REV. B

–10–

8-BIT MICROPROCESSOR SYSTEMS

Figure 22 shows an interface circuit for the AD7845 to the

8085A 8-bit microprocessor. The software routine to load data

to the device is given in Table IV. Note that the transfer of the

12 bits of data requires two write operations. The first of these

loads the 4 MSBs into the 7475 latch. The second write opera-

tion loads the 8 LSBs plus the 4 MSBs (which are held by the

latch) into the DAC.

Figure 22. 8085A Interface

Table IV. Subroutine Listing for Figure 22

2000 LOAD DAC: LXI H,#3000 The H,L register pair

are loaded with latch

address 3000.

MVI A,#“MS” Load the 4 MSBs of

data into accumulator.

MOV M,A Transfer data from

accumulator to latch.

INR L Increment H,L pair to

AD7845 address.

MVI A,#“LS” Load the 8 LSBs of

data into accumulator.

MOV M,A Transfer data from

accumulator to DAC.

RET End of routine.

MICROPROCESSOR INTERFACING

16-BIT MICROPROCESSOR SYSTEMS

Figures 19, 20 and 21 show how the AD7845 interfaces to

three popular 16-bit microprocessor systems. These are the

MC68000, 8086 and the TM32010. The AD7845 is treated as

a memory-mapped peripheral to the processors. In each case, a

write instruction loads the AD7845 with the appropriate data.

The particular instructions used are as follows:

MC68000: MOVE

8086: MOV

TMS32010: OUT

Figure 19. AD7845 to MC68000 Interface

Figure 20. AD7845 to 8086 Interface

Figure 21. TMS32010

AD7845

REV. B –11–

Figure 23 and 24 are the interface circuits for the Z80 and

MC6809 microprocessors. Again, these use the same basic

format as the 8085A interface.

Figure 23. AD7845 to Z80 Interface

Figure 24. MC6809 Interface

DIGITAL FEEDTHROUGH

In the preceding interface configurations, most digital inputs to

the AD7845 are directly connected to the microprocessor bus.

Even when the device is not selected, these inputs will be con-

stantly changing. The high frequency logic activity on the bus

can feed through the DAC package capacitance to show up as

noise on the analog output. To minimize this digital feedthrough

isolate the DAC from the noise source. Figure 25 shows an

interface circuit which uses this technique. All data inputs are

latched from the busy by the CS signal. One may also use other

means, such as peripheral interface devices, to reduce the digital

feedthrough.

Figure 25. AD7845 Interface Circuit Using Latches to

Minimize Digital Feedthrough

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AD7845BQ

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Digital to Analog Converters - DAC 12B CMOS Multiplying

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