OPA890 datasheet(17/34 Pages) NSC

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2pR

FCF

GBP

4pR

FCD

*3dB +

GBP

2pR

FCD

OPA890

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SBOS369B – MAY 2007 – REVISED DECEMBER 2009

Driving a light load, the OPA890 can output ±4V over

Notice that most of the error occurs mainly at the first

±5V supplies. Setting the reference voltage to –5V

codes (0, 1, 2); excluding these codes from the

results in an output voltage swing from 0V to 5V. In

analysis

yields

the

following

results,

shown

order to optimize the OPA2890 operation for this

Table 1.

application, the supply voltages have been adjusted

so that the output voltage swing is balanced around

Table 1. DC Accuracy vs Code

mid-supply of the amplifier. Note that as a result of

TOTAL ERROR DUE TO

the internal architecture of the multiplying DAC, the

CODES

VOS and IB

IOUT1 output is not high impedance. The IOUT1 output

All codes

3.9LSB

resistance is between 4.5k

Ω and 22.1kΩ (excluding

Excluding code 0

2.5LSB

code 000h) for a 10k

Ω nominal VREF input resistance.

Excluding codes 0 and 1

2LSB

IOUT1 output resistance changes are directly related to

the code change. This low impedance has multiple

Excluding codes 0, 1, and 2

1.83LSB

effects when a bipolar technology amplifier is used.

Note that 1LSB = 1.221mV in the example shown in

Some of these effects are:

Figure 48

•

The noise gain of the amplifier changes for each

If more precision is required while maintaining the ac

code.

performance, a FET-input amplifier (such as the

•

The output offset voltage of the amplifier changes

OPA656 or the THS4631) is a good alternative.

for each code, because of the input offset voltage.

Figure

shows

single-ended

output

drive

•

The input bias current cannot be cancelled. The

implementation. In this circuit, only one side of the

effects of the input bias current can be reduced,

complementary output drive signal is used. A dual

but not eliminated, thereby affecting the total

amplifier, such as the OPA2890, provides both output

output offset voltage of the amplifier with each

drivers for the DAC7822. If even lower quiescent

code.

current is needed, the OPA2889 can be used instead,

•

The noninverting pin of the amplifier must be tied

with minor modifications. The diagram shows the

to ground and cannot be used to create a dc

signal output current connected into the virtual ground

offset on the output amplifier, as is the case for

summing junction of the OPA890, which is set up as

the transimpedance amplifier.

a transimpedance stage or I-V converter. The unused

The following analysis excludes the input offset

current output of the DAC is connected to ground.

current.

The dc gain for this circuit is equal to RF. At high

frequencies, the DAC output capacitance produces a

The total output offset voltage variations because of

zero in the noise gain for the OPA890 that may cause

code changing in the DAC can be expressed as:

peaking in the closed-loop frequency response. CF is

ΔVOSO = +ΔNG {[(RF

ROUT1) – RS] + VOS}

added across RF to compensate for this noise gain

peaking. To achieve a flat transimpedance frequency

Where:

response, the pole in the feedback network should be

4.5k

Ω ≤ ROUT1 ≤ 22.1kΩ

set to:

RF = 10kΩ

Using the previous values, the variation of the parallel

(2)

combination of RF and ROUT1 can be constrained to:

4.19k

Ω ≤ (RF

ROUT1) ≤ 6.88kΩ. In order to optimize

which

gives

closed-loop

transimpedance

the bias current cancellation, we select RS to be the

bandwidth, f–3dB, of approximately:

average of those limiting numbers, or RS = (6.88kΩ +

4.19k

Ω)/2 = 5.56kΩ.

(3)

Looking at the variation for each code, the total error

(when including all codes) is ~3.9LSB for the

Using the DAC7822 internal output capacitance of

OPA890.

25pF gives a feedback capacitance (CF) of 2.5pF and

an 8.8MHz bandwidth.

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