LTC1067-50CS datasheet(14/20 Pages) AD

14 / 20 page

LTC1067/LTC1067-50

APPLICATIONS INFORMATION

Many applications are more concerned with the negative

output swing than the positive output swing. Interfacing to

an ADC running on a single 5V supply with a 4.096

reference voltage is a standard example. The LTC1067 or

LTC1067-50 will easily reach the 4.096V level for a full-

scale reading. The issue is how close does the output go

to ground. The further the output is from ground, the more

codes that are essentially lost. The previous example

demonstrated that the lowest output voltage would be

about 250mV, although, as is shown below, 15mV is

achievable.

To achieve a lower negative output swing voltage, the

AGND voltage must be adjusted down below the midpoint.

The AGND voltage is determined by two equal, on-chip

resistors. These resistors are typically 15k each. While the

ratio of these two resistors is tightly matched, the absolute

value of the resistors is not tightly controlled. Adjusting

the AGND voltage by simply adding an external resistor

can be done, but caution must be exercised.

In Figure 13, a resistor is used to adjust the AGND voltage

for use with a 5V powered ADC with a full-scale input of

4.096V. The resistor value was chosen carefully to assure

that a 4.096V input signal to the filter yields a full-scale

reading from the ADC and a 0V input signal gives the

lowest possible value (15mV for the LTC1067 and 10mV

for the LTC1067-50). The circuit works well over tempera-

ture and part variations. For this application, the 5V supply

must be above 4.75V.

LOAD RESISTANCE (k

Ω TO V–)

5.0

4.5

4.0

3.5

3.0

2.5

1067 F11

12 14 16 18 20

LTC1067-50

LTC1067

Figure 11. LTC1067/LTC1067-50 Positive Output Voltage

Swing vs Load Resistance, 5V Supply

the output of the first section is near the positive rail

(operating modes invert the signal). The output of the first

stage will saturate at about 250mV (typical for 5V supply)

from positive supply. The output from the second stage

will be 250mV from the negative supply rail (assuming

inversion again) even though the op amp’s output is

capable of swinging to within 15mV.

The positive output voltage swing being less than the

negative swing, coupled with the AGND potential set at the

midpoint of the supplies and inverting of the signal, yields

the following equation for peak-to-peak output swing:

VP-P Swing = (V + – V –) – 2(V+ – VPOSITIVE SWING)

LOAD RESISTANCE (k

Ω TO V–)

3.3

3.0

2.7

2.4

2.1

1.8

1.5

1067 F12

12 14 16 18 20

LTC1067-50

VS = 3V

LTC1067

VS = 3.3V

Figure 12. LTC1067/LTC1067-50 Positive Output Voltage

Swing vs Load Resistance, 3.3V/3V Supplies

0.1

µF

1067 F13

64.9k

LTC1067

LTC1067-50

V+

LPA

BPA

HPA/NA

INV A

CLK

AGND

V–

LPB

BPB

HPB/NB

INV B

(4.75VMIN)

µF

Figure 13. Power and AGND Connections for

5V ADC with 4.096V Full Scale

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