SNAS136M – APRIL 2001 – REVISED MARCH 2013
The reference bias circuit of Figure 31 is very simple and the performance is adequate for many applications.
However, circuit tolerances will lead to a wide reference voltage range. Better reference stability can be achieved
by driving the reference pins with low impedance sources.
The circuit of Figure 32 will allow a more accurate setting of the reference voltages. The upper amplifier must be
able to source the reference current as determined by the value of the reference resistor and the value of (VRT −
VRB). The lower amplifier must be able to sink this reference current. Both amplifiers should be stable with a
capacitive load. The LM8272 was chosen because of its rail-to-rail input and output capability, its high current
output and its ability to drive large capacitive loads.
The divider resistors at the inputs to the amplifiers could be changed to suit the application reference voltage
needs, or the divider can be replaced with potentiometers or DACs for precise settings. The bottom of the ladder
(VRB) may be returned to ground if the minimum input signal excursion is 0V.
VRT should always be more positive than VRB by the minimum VRT - VRB difference in Electrical Characteristics to
minimize noise. While VRT may be as high as the VA supply voltage and VRB may be as low as ground, the
difference between these two voltages (VRT − VRB) should not exceed 2.3V to prevent waveform distortion.
The VRM pin is the center of the reference ladder and should be bypassed to a quiet point in the ground plane
with a 0.1 µF capacitor. DO NOT leave this pin open and DO NOT load this pin with more than 10µA.
470: 1 PF
+ 10 PF
+ 10 PF
1 4 12
2 5 8 11
DR GND CLK
Figure 32. Driving the reference to force desired values requires driving with a low impedance source.
THE ANALOG INPUT
The analog input of the ADC08200 is a switch followed by an integrator. The input capacitance changes with the
clock level, appearing as 3 pF when the clock is low, and 4 pF when the clock is high. The sampling nature of
the analog input causes current spikes at the input that result in voltage spikes there. Any amplifier used to drive
the analog input must be able to settle within the clock high time. The LMH6702 and the LMH6628 have been
found to be good amplifiers to drive the ADC08200.
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