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> A/D and
D/A Conversion/Sampling Circuits > APP
2398
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> Amplifier
and Comparator Circuits > APP 2398
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App notes
> Automotive
> APP 2398

Sep 29, 2003

Keywords: 3V DACs, level shifting, +/-10V, 10V,
3V output, industrial DAC, bipolar, resistor networks, gain
setting resistor, amplifier, op amp, digital pot
Related
parts

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REFERENCE DESIGN 2398

INCLUDES: Tested
circuit Schematic Description Test
data

3V DACs Used in ±10V Applications

Abstract: Many
modern systems have the majority of their electronics powered
by 3.3V or lower, but must drive external loads with ±10V, a
range that is still very common in industrial applications.
There are digital to analog converters (DACs) available that
can drive loads with ±10V swings, but there are reasons to use
a 3.3V DAC and amplify the output voltage up to ±10V.

Introduction
Modern logic systems that use 3.3V
supplies must sometimes run industrial applications which
require voltages of ±10V, such as PLC,
transmitters, and motor control. One solution is to choose a
digital to analog
converter (DAC)
that can deliver ±10V swings, but an alternative is to use a
3.3V DAC and amplify the output voltage
up to ±10V. Examples:

3.3V DACs may have higher levels of desired logic
integration than their ±10V counterparts.

3.3V DACs tend to have higher speed logic interfaces,
freeing the microcontroller for other tasks.

The DAC may be integrated into a larger chip, powered
from 3.3V (such as a microcontroller) and not capable of
±10V output swings.

The external loads may have output current
drive requirements or capacitive loads that cannot be
handled by the 3.3V ±10V DAC.

Circuit Overview
The circuit's block diagram is shown
in Figure 1a. It consists of five main blocks: DAC,
Reference, Offset Adjust, Reference Buffer, and Output Buffer.

The DAC provides the digital-code-to-voltage
translation, relative to the Reference. Offset Adjust provides
the ability to offset the DAC's unipolar transfer
function to create a bipolar output and the ability to
calibrate the 0V output point. The Reference Buffer provides
load isolation for the Reference and the Offset Adjust. The
Output Buffer sums in the offset adjustments and provides the
required gain to increase the output swing to the desired
magnitude. Additionally the Output Buffer provides the ability
to drive a load.

Circuit Description
The circuit in Figure 1 and
Figure 1a illustrates how a 3.3V powered 16-bit DAC can be
buffered to achieve output swings of ±10V. The DAC (U2) has a
0 to 2.5V output range and is connected to the non-inverting
input of op
amp U3. This input has a non-inverting gain of (1 + 26.25k
/ 3.75k) or 8. The inverting input of the op amp is connected
to +1.429V created by the reference and the resistor divider
network. This input has a gain of -(26.25k / 3.75k) or -7. The
output of this circuit with the DAC programmed to minus full
scale (0V) is = (0 x 8) - (7 x 1.429) = -10V. With the DAC
programmed to full scale (2.5V), the final output is = (2.5 x
8) - (7 x 1.429) = +10V.

Figure
1.

Figure
1a.

The circuit consists of the following parts:

U1: MAX6133A, 2.5V Reference

U2: MAX5443, 16 bit, 3.3V Supply, Serial
DAC

U3 & U4: OP07A, Precision Op Amp, ±15V supplies

U5: MAX5491A, Precision Resistor Network with ESD
Protection, 3:4 Ratio

U6: MAX5491A, Precision Resistor Network with ESD
Protection, 1:7 Ratio

U7: MAX5423, 100K, 256 Tap, Nonvolatile, Digital
Potentiometer

Reference
The 2.5V reference voltage is used by the
DAC as its reference voltage and to generate the +1.429V. It
is important that both these functions use the same reference
voltage. Any error in the tracking between these two voltages
will affect the outputs zero offset voltage. A common error
will only affect the outputs full-scale gain, which is usually
a much less critical parameter. 2.5V was chosen for the main
reference voltage since it is a very common value and will
work with both 3.3V and 5V power supplies. The MAX6133A was
chosen because it has very good performance in a small µMAX®
package. The critical parameters for this part are Output
Voltage Accuracy (±0.06%), Temperature
Coefficient (7ppm / °C), and Long Term Stability (145ppm /
1kHrs).

DAC
The most critical parameter in industrial control
applications is zero offset error. The unipolar output MAX5443
in the example has ±2 LSBs of offset error and ±10 LSBs of gain
error. This is more than adequate for most applications.
In order to translate the DAC's output to bipolar offset
circuitry is used do to shift the DACs zero code (minus full
scale) to -10V and its mid scale code to 0V. Instead of
getting less than ±2 LSB
of zero code error specified at 0V the DAC's mid-scale code
error is the sum of the zero code error and the gain error at
mid-scale. This may not be acceptable so the example features
a digital
potentiometer to recalibrate the zero output.

Op Amp
Op amp U4 is used as a reference buffer between
the reference divider resistors (U5) and op amp U3's gain
resistors. If more than one DAC is in the system they can all
share this buffered output. Op amp U3 provides the
amplification and offsetting of the DAC voltage. Selection of
the op amp and configuration of this stage will depend on the
requirements of the load. The following load specifications
need to be considered:

Maximum voltage swing

Maximum drive current

Capacitance loading

Short circuit protection

ESD protection

The OP07A in the example can
supply ±10V at 10mA to the load. The R1 and C2 network allows
the op amp to drive large capacitive loads.

The op amp
parameters that will affect system accuracy are VOS
(25µV) and IOS (2nA). The effects of IB
(2nA) are negated by R3 and R4. By making the equivalent resistance
in each input of the op amp the same, the effects of
IB cancel out. The slew rate of the system will be
limited by the 0.1V/µS slew rate of the OP07A. This is usually
not a problem in industrial control applications.

Resistor Networks
The U5 resistor network (3:4 ratio)
divides the +2.5V reference down to +1.429V and the U6
resistor network (1:7 ratio) sets op amp U3's gain. The
critical parameters here are initial ratio error (0.035%) and
ratio temperature coefficient (5ppm / °C). The MAX5491 was
chosen over conventional networks because of its ±2kV of ESD
protection. This is important since one side of U6 may go off
board and be subjected to ESD discharges.

Digital Potentiometer
The MAX5423 256 tap digital
potentiometer is used to adjust the systems zero offset error.
This part has a nonvolatile
memory so offset values are retained even with the power
shut off. The resistor network formed by U7, U5, and R2 were
chosen to provide approximately ±100 LSBs of adjustment at 0V.

Analysis
A PSPICE sensitivity analysis was performed
on this circuit. The results show the initial zero output
error of 13 LSBs max. This error source
can be calibrated out using the Digital Pot. The temperature
analysis shows a total offset error of 0.126 LSBs / °C. With a
100 deg temp variation this would be 12.6 LSBs of offset
error. This is adequate for most applications.

Table
1. Sensitivity Analysis, Zero Output, Intitial Tolerance
Errors In LSBs at output

Ref Design

Component

Description

Error Source

Error Value

Error Units

Sensitivity

Sensitivity Units

Output Error (LSBs)

U1

MAX6133A

2.5V Ref

Output Accuracy

0.06

%

-2.74E - 04

LSBs/%

0.00

U2

MAX5443

16 bit DAC

Gain Error

5

LSBs

1.00E + 00

LSB/LSB

5.00

U3

OP07A

OpAMp

VOS

25

µV

-2.62E + 04

LSB/V

0.66

U3

OP07A

OpAMp

IOS

2

nA

8.55E + 07

LSB/A

0.17

U3

OP07A

OpAMp

IB

2

nA

1.08E + 06

LSB/A

0.00

U4

OP07A

OpAMp

VOS

25

µV

-2.29E + 04

LSB/V

0.57

U4

OP07A

OpAMp

IOS

2

nA

1.68E + 08

LSB/A

0.34

U4

OP07A

OpAMp

IB

2

nA

8.10E + 03

LSB/A

0.00

U5

MAX5491A

Res Network

Ratio Tolerance

0.035

%

1.40E + 02

LSB/%

4.90

U6

MAX5491A

Res Network

Ratio Tolerance

0.035

%

4.09E + 01

LSB/%

1.43

Total

13.07

Table 2. Sensitivity
Analysis, Zero Output, Temperature Errors In LSBs / Deg C at
output

Ref Design

Component

Description

Error Source

Error Value

Error Units

Sensitivity

Sensitivity Units

Output Error (LSB / °C)

U1

MAX6133A

2.5V Ref

Output Temp Co

7

ppm/°C

2.74E - 04

LSBs/%

1.92E - 07

U2

MAX5443

16 bit DAC

Gain Temp Co

0.1

ppm/°C

5.00E - 02

LSB/%

5.00E - 07

U3

OP07A

OpAMp

VOS Temp Co

0.6

µV/°C

-2.62E + 04

LSB/V

1.57E - 02

U3

OP07A

OpAMp

IOS Temp Co

25

pA/°C

8.55E + 07

LSB/A

2.14E-03

U3

OP07A

OpAMp

IB Temp Co

25

pA/°C

1.08E + 06

LSB/A

2.70E - 05

U4

OP07A

OpAMp

VOS Temp Co

0.6

µV/°C

-2.29E + 04

LSB/V

1.38E - 02

U4

OP07A

OpAMp

IB Temp Co

25

pA/°C

1.68E + 08

LSB/A

4.20E - 03

U4

OP07A

OpAMp

IB Temp Co

25

pA/°C

8.10E + 03

LSB/A

2.02E - 07

U5

MAX5491A

Res Network

Ratio Temp Co

5

ppm/°C

1.40E + 02

LSB/%

7.00E - 02

U6

MAX5491A

Res Network

Ratio Tamp Co

5

ppm/°C

4.09E + 01

LSB/%

2.05E - 02

Total

1.26E-01

µMAX
is a registered trademark of Maxim Integrated Products,
Inc.

Related parts

MAX5423

256-Tap, Nonvolatile,
SPI-Interface, Digital Potentiometers

MAX5443

+3V/+5V, Serial-Input,
Voltage-Output, 16-Bit DACs

Free
samples

MAX5491

Precision-Matched
Resistor-Divider in SOT23

Free
samples

MAX6133

3ppm/°C, Low-Power,
Low-Dropout Voltage Reference

Free
samples

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Download,
PDF format (44kB)
APP 2398: Sep 29, 2003

AN2398, AN 2398, APP2398,
Appnote2398, Appnote 2398

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