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这两个脚位的电路原理来了（PCM63的），供参考。PCM1704有些许差异：
REFERENCE AND SERVO AMPLIFIER
An onboard reference reduces the need for external components and enhances system performance by tracking the temperature changes of the DAC.
It is one of the most critical elements since the basic accuracy of the converter is directly dependent on it.
A subsurface zener diode is at the heart of the reference and provides a highly stable, Iow noise, voltage source. The bias current is generated from the zener itself and being held at a constant value, enhances reference voltage stability.
Since the zener is used to bias itself, a start-up circuit is required.
This is shown in Figure 8, where the N channel epitaxial FET turns on at start-up and supplies the base current to the biasing transistor.
This in turn, provides the zener current via a lateral PNP mirror circuit.
As the zener turns on and the voltage on the gate of the FET increases, the FET is pinched off and is effectively removed.
A Vbe multiplying circuit comprising Q408, Q409, R406 and R407 is required to compensate for the overall negative temperature coefficient of the circuit.
The output of the circuit is a current, constant to within +/-25 ppm over the temperature range.
This current is supplied to the servo amplifier, the output of which (VREF) feeds all the DAC current sources.
It can be seen that the DAC output current can be increased or decreased with respect to the reference current by trimming R503 or R504.
Cascode Q508 simulates the effect of the bit switch associated with the true current sources and, therefore, ensures better tracking.
Lateral PNP transistor Q506 prevents any possible latch-up caused by unusual power supply sequencing.
The base of output transistor Q505 is brought out to a pad for decoupling.
The external decoupling capacitor serves a four-fold purpose: to compensate the amplifier, to improve power supply rejection, to reduce noise and to maintain a low impedance on VREF.
REFERENCE AMPLIFIER
The trans-resistance, reference amplifier shown in Figure 9 provides a stable reference voltage at the emitter of Q603.
This voltage is developed by the reference current flowing through trimmable resistor R605. The reference current is generated in an identical fashion to the bit current and therefore tracks any variations.
Trimming of R605 or the bipolar offset resistor, R606, allows bipolar zero to be adjusted up or down.
The offset contributed decoupling capacitor compensates to the converter output.
The three analog components:
current reference, servo amplifier and reference amplifier, are placed along one edge of the die as far removed from the critical current sources as possible.
This helps to minimize the effects of unequal heating.
Aiso, since the heat from these sources is constant, any remaining effect can be eliminated during the trim phase.
Placing these components on the chip, not only removes an enormous burden from the user, but gives improved performnce because of their excellent tracking over temperature and process variations.
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楼主
发表于 2021-2-12 13:38
