SP9841 datasheet(27/32 Pages) SIPEX

27 / 32 page

293

grammed gain. Please refer to the THD versus

Frequency plot, which was generated by termi-

nating a 600 Ohm source with 150 Ohms to

ground, then into this circuit. For the best gain

linearity versus code, use the largest (lowest

series impedance) coupling capacitor available,

or externally buffer the input.

Figure 13 shows an external op amp with an

inverting programmable gain. In this circuit the

maximum output swing at the DAC occurs at the

maximum circuit gain. Thus, the headroom re-

striction at the DAC output applies at the maxi-

mum gain, which, for rail–to–rail outputs (V

REFL

= 2.5V or V

DD/2) should be greater than 4.3. By

making the programmable gain range large, this

circuit can be used to provide rail–to–rail out-

puts even at the lower gains. This circuit has

been ratioed to provide exact integer gain incre-

ments for every increase in 25 codes, over the

range of –1 to –11. This large range of gain

comes at a slight cost — the output offset of the

DAC amplifier will be gained up by –5.12 times

at SIG OUT. If this is a problem, a second DAC

channel can be set up with a programmable DC

offset adjustment with its output summed through

a large resistor into the OP–491 inverting termi-

nal. Note that when RTRIMRANGE is set up for

only unity gain change range as in Figure 12,

only –0.5 times the DAC output offset will

appear at SIG OUT.

Another application for the circuits of both

Figure 10 and 13 could be to force precise gains

from circuits made from imprecise resistors. By

restricting the programmable gain range to

±2%

(by setting R

TRIMRANGE to be 100 times RF), the

resistors could be 1% values and the program-

mable gain resolution would increase to better

than 12–bits (0.0156%). In this case, only 1% of

the DAC output offset voltage would appear at

SIG OUT.

Figure 14 shows a window comparator and two

channels of programmable-gain input. While

the input signal is shown as AC–coupled, DC

signals of up to rail–to–rail amplitude could be

measured by setting the attenuation at the signal

Figure 13. Adjustable Gain of External Inverting Opamp Circuit, V

OUT = Rail–to–Rail.

–

SIG OUT

1/8 of SP9841

1/4 of OP–491

+5V

Set R

TRIMRANGE for desired gain-trim range.

For R

TRIMRANGE = 1.5kOhms:

Code 0, A

V = -1

Code 25, A

V = -2

Code 75, A

V = -4

Code 175, A

V = -8

Code 225, A

V = -10

Code 250, A

V = -11

Gain resolution = 4%

–

7.68kOhms

RTRIMRANGE

1.5kOhms

REFL

= Up to VDD/2

1.25V

P-P

2.2µF

RGAIN

7.68kOhms

REFL

+5V

, D = 0 to 255

TRIMRANGE

=−

− 















128

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