OPA827AI datasheet(18/24 Pages) TI

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OPA827 USED AS AN I/V CONVERTER

OUT =

-V

CODE

REF

65536

(3)

DAC8811

OUT

GND

REF

+10V

OUT

-10V

+10V

OUT

10kW

5kW

OPA827

SBOS376E – NOVEMBER 2006 – REVISED SEPTEMBER 2008 ..................................................................................................................................... www.ti.com

The DAC output impedance as seen looking into the

IOUT terminal changes versus code. The low offset

The OPA827 series of operation amplifiers have low

voltage

the

OPA827

minimizes

the

error

current noise and offset voltage that make these

propagated from the DAC.

devices a great choice for an I/V converter. The

DAC8811 is a single channel, current output, 16-bit

For a current-to-voltage design (see Figure 49), the

digital-to-analog converter (DAC). The IOUT terminal

DAC8811 IOUT pin and the inverting node of the

of the DAC is held at a virtual GND potential by the

OPA827 should be as short as possible and adhere

use of the OPA827 as an external I/V converter op

to good PCB layout design. For each code change on

amp. The R-2R ladder is connected to an external

the output of the DAC, there is a step function. If the

reference input (VREF) that determines the DAC

parasitic capacitance is excessive at the inverting

full-scale current. The external reference voltage can

node, then gain peaking is possible. For circuit

vary in a range of –15V to +15V, thus providing

stability, two compensation capacitors, C1 and C2(4pF

bipolar IOUT current operation. By using the OPA827

to 20pF typical) can be added to the design.

as an external I/V converter in conjunction with the

Some

applications

require

full

four-quadrant

internal DAC8811 RFB resistor, output voltage ranges

multiplying capabilities or a bipolar output swing. As

of –VREF to +VREF can be generated.

shown in Figure 49, the OPA827 is added as a

When using an external I/V converter and the

summing amp and has a gain of 2x that widens the

DAC8811 RFB resistor, the DAC output voltage is

output span to 20V. A four-quadrant multiplying circuit

given by Equation 3.

is implemented by using a 10V offset of the reference

voltage to bias the OPA827.

NOTE: CODE is the digital input into the DAC.

Figure 49. I/V Converter

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