X79000 Datasheet, PDF(16/18 Page) Intersil Corporation – NV DAC with Selectable Output Range and Memory

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X79000 Datasheet, PDF (16/18 Pages) Intersil Corporation – NV DAC with Selectable Output Range and Memory

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X79000, X79001, X79002

APPLICATIONS INFORMATION

Remote sensing

The output opamp included in the X79000 and X79001 is

normally configured with a gain of +1, and since the

inverting terminal is available externally, can be used for

remote load sensing (see Figure 3). This configuration is

useful for high accuracy applications which may draw

significant current from the DAC output with a finite

impedance from the DAC to the load. The inverting

terminal must be brought as close as possible to the

load, and there must be very low differential in the

ground potentials of the two circuits.

Output Voltages Greater than 3.025V

The opamp output (Vbuf) can drive up to Â±1mA and stay

within 150mV of ground and the VCC supply. Normally, if

the opamp is configured with a gain of +1, Vbuf is limited

to 3.10V max, which is the limit of the DAC Vout. If gain

is added to the opamp feedback loop, then Vbuf can

provide a higher output voltage, up to 4.85V with

VCC = 5.00V. Figure 4 shows a circuit with a gain of +2

that is configured for 4.84V max Vbuf, with VH internally

set to 2.42V (VH2, VH1, VH0 set to 1,0,0). Care must be

taken when increasing the maximum Vbuf output,

however, in this example VCC may have a range of Â±5%,

or 4.75V to 5.25V. The maximum Vbuf can be expected

to reach and stay within specifications is 4.75V -

150mV = 4.600V. If the output offset of the DAC is

included (22mV x 2, worst case), then the max output will

be 4.84V + 0.044V = 4.884V. The designer has the

option of either realizing that the DAC may miss the

higher codes, or change the amplifier gain to a value less

than 2 (or 4.60/2.42 = 1.90, for this example) to keep all

codes and reduce the maximum Vbuf output.

Using the VH and VL pins for multiplying functions

When a time-varying waveform is applied at either

reference input pin, the output reflects a scaled version of

that waveform (see Figure 5). This waveform will follow

the DAC output voltage equation when applied to VH:

Vbuf = [(VH - VL)(n/4095)] + VL, n = 0 to 4095

(excluding DAC, Reference scaling and opamp errors)

This shows that the input range for the waveform is

limited to VL on the low side, and by the Vout range

(3.10V) on the high side. The output is scaled by the

DAC setting to allow for gain control. The maximum

output voltage can be increased as shown in Figure 4

using the opamp and Vbuf output. It is advisable that the

VH pin be driven by a low impedance source for optimal

AC performance. The minimum bandwidth of the circuit

is 50kHz over all specified voltage range, temperature

and output loading configurations.

Note that it is possible to use the VL pin in the same

fashion, with VH fixed, but the resulting waveform will

have a slightly different transfer function:

Vbuf = VH - (VH - VL)[(4095-n)/4095], n = 0 to 4095

Alternatively, the VL input could include a variable

reference, such as a temperature sensor, or a shunt

reference connected between VH and VL, which would

fix their differential (the configuration register must be

set for external VH and VL references). This provides a

DAC output which varies proportional to temperature,

yet can be set to an arbitrary voltage by the DAC for

biasing applications.

FN8147.0

March 17, 2005

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