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APPLICATION INFORMATION

Stability Circuit

GND

I

1

OUT

R

FB

V

REF

V

REF

V

DD

V

DD

V

OUT

I

2

OUT

C

1

U1

U2

Amplifier Selection

DAC7821

SBAS365B – OCTOBER 2005 – REVISED JULY 2007

For a current-to-voltage design (see Figure 27), the DAC7821 current output (IOUT) and the connection with the

inverting node of the op amp should be as short as possible and according to correct printed circuit board (PCB)

layout design practices. For each code change, there is a step function. If the gain bandwidth product (GBP) of

the op amp is limited and parasitic capacitance is excessive at the inverting node, then gain peaking is possible.

Therefore, for circuit stability, compensation capacitor C1 (1pF to 5pF typ) can be added to the design, as shown

in Figure 27.

Figure 27. Gain Peaking Prevention Circuit with Compensation Capacitor

There are many choices and many differences in selecting the proper operational amplifier for a multiplying DAC

(MDAC). Making the analog signal out of the MDAC is one critical aspect. However, there are also other issues

to take into account such as amplifier noise, input bias current, and offset voltage, as well as MDAC resolution

and glitch energy. Table 1 and Table 2 suggest some suitable operational amplifiers for low power, fast settling,

and high-speed applications. A greater selection of operational amplifiers can be found at www.ti.com/amplifer.

Table 1. Suitable Precision Operational Amplifiers from Texas Instruments

IQ

TOTAL

TOTAL

PER

SLEW

OFFSET

SUPPLY

SUPPLY

CHANNEL

GBW

RATE

DRIFT

IIB

CMRR

VOLTAGE

VOLTAGE

(max)

(typ)

(typ)

(typ)

(max)

(min)

PACKAGE/

PRODUCT

(V) (min)

(V) (max)

(mA)

(MHz)

(V/

μs)

(

μV/

°C)

(pA)

(dB)

LEAD

DESCRIPTION

Low Power

SOT5-23,

12V, CMOS, Rail-to-Rail I/O,

OPA703

4

12

0.2

1

0.6

4

10

70

PDIP-8,

Operational Amplifier

SOIC-8

0.05

μV/

°C (max),

SOT5-23,

Single-Supply CMOS

OPA735

2.7

12

0.75

1.6

1.5

0.01

200

115

SOIC-8

Zero-Drift Series Operational

Amplifier

Low Power, Single-Supply,

SOT5-23,

Rail-To-Rail Operational

OPA344

2.7

5.5

0.25

1

1

2.5

10

80

PDIP-8,

Amplifiers MicroAmplifier

SOIC-8

Series

SC5-70,

1MHz, 45

μA, Rail-to-Rail I/O,

OPA348

2.1

5.5

0.065

1

0.5

2

10

70

SOT5-23,

Single

SOIC-8

Op Amp

PDIP-8,

High Precision Operational

OPA277

4

36

0.825

1

0.8

0.1

1000

130

SOIC-8,

Amplifiers

SON-8

Fast Settling

High-Speed, Single-Supply,

MSOP-8,

Rail-to-Rail Operational

OPA350

2.7

5.5

7.5

38

22

4

10

76

PDIP-8,

Amplifiers MicroAmplifier

SOIC-8

Series

e-trim 20MHz, High

MSOP-8,

OPA727

4

12

6.5

20

30

0.6

500

86

Precision CMOS Operational

SON-8

Amplifier

PDIP-8,

High Precision, Low Noise

OPA227

5

36

3.8

8

2.3

0.1

10000

120

SOIC-8

Operational Amplifiers

12

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