Examples / TI op amp handbook / References

Presettable voltage source

SBOA092B page 52, Presettable Voltage Source: the cell Eref from the node RI and RO share to the inverting input, RO (a decade box) on to the output, RI to ground, and the non-inverting input on ground. The op amp is a TLC265x.

E_O = (R_I + R_O) / R_I x Eref

The circuit

The schematic is drawn by copperhead’s drafting engine from this circuit’s netlist, with KiCad’s own library symbols, and it opens in KiCad as figure/presettable_voltage_source.kicad_sch. The op amp is KiCad’s generic one, since the handbook’s are ideal, and each terminal is a test point named as the program names it. KiCad reads back from the sheet exactly the connections the circuit has; draw_figures.py refuses to write one that does not.

the interconnect view, fang's own projection — The interconnect view, fang's own projection

The interconnect view is fang’s own projection. It names the parts as the program does, so it reads against the code below.

What the program says

The inverting input sits at ground, so the shared node stands at +Eref and R_I carries Eref / R_I, all of it from the output through R_O. The figure sizes R_I at 1000 x Eref ohms, which makes that current 1 mA whatever the cell is: each ohm on the decade box is a millivolt of E_O above Eref. The handbook does not say so; the program records it as a calculation (scale) and a constraint (i_set = 1 mA).

Recorded choices: cell, a Weston cell at 1.0183 V, so R_I = 1018.3 Ω; dial, R_O at 8981.7 Ω for E_O = 10.000 V; and decade_box, the decade box modelled as a potentiometer with its wiper on its far end, so a run can turn the dial. The op amp has a chopper’s 1 µV offset.

What the simulation found

out/simulation.txt:

Run	R_O	Measured	Claimed
`ten_volts`, E_O	8981.7 Ω	10.000 V	10 V (`e_out`), holds
`ten_volts`, current in R_I		1 mA	1 mA (`i_set`), holds
`ten_volts`, cell current		1e-17 A	0 (`i_cell`) ± 1 nA, holds
`five_volts`, E_O	3981.7 Ω	5.000 V	5 V, holds
`dial_at_zero`, E_O	0 Ω	1.0183 V	Eref, holds

The outputs are held to 100 ppm: a noise gain near 10 costs 10 ppm of loop-gain error and the offset another 1 ppm. The lowest output the circuit gives is Eref itself.

Running it

fang check examples/ti_opamp_handbook/references/presettable_voltage_source/presettable_voltage_source.py
python examples/regenerate.py ti_opamp_handbook/references/presettable_voltage_source   # needs ngspice

The whole program

"""The presettable voltage source, SBOA092B page 52.
Show 20 more lines

    E_O = (R_I + R_O) / R_I x Eref

The cell's - terminal is on the - input, which the loop holds at ground, so
its + terminal, the node R_I and R_O share, stands at +Eref. R_I then carries
Eref / R_I to ground, all of it from the output through R_O, and the output
settles at Eref (1 + R_O / R_I). The cell carries only what the - input draws.

The figure sizes R_I as "1000 x Eref" ohms. That makes the current through it
1 mA whatever the cell is, so the decade box reads directly: every ohm of R_O
is a millivolt of E_O above Eref. The program records that as a calculation
of its own (`scale`), since the handbook does not say it.

Open: the cell's value, and R_O's dial. `cell` records a Weston cell,
1.0183 V, so R_I is 1018.3 Ohm. `dial` sets R_O to 8981.7 Ohm, for 10.000 V.
The decade box is drawn as a variable resistor, so the program models it as a
potentiometer with its wiper on one end (`decade_box`), and a run can turn it:
3981.7 Ohm for 5 V, and 0 Ohm for Eref itself. The op amp is the TLC265x the
figure names, with a chopper's 1 uV offset.
"""

import sys
from decimal import Decimal
from pathlib import Path

# The handbook's shared parts and bench live in the folder above the sections.
sys.path.insert(0, str(Path(__file__).resolve().parents[2]))

from fang.lang import A, Ohm, Parameter, System, V, mA, require, uV
from fang.parts import Resistor
from fang.rationale import Calculates, Chooses, Cites
from fang.simulation import OperatingPoint

from handbook import (
    Bench,
    Cell,
    Claim,
    Ground,
    OpAmp,
    Potentiometer,
    Run,
    Terminal,
    equals,
    over,
    product,
    ratio,
    total,
)


class PresettableVoltageSource(System):
    """Eref from the R_I/R_O node to the - input; R_O on to the output, R_I to ground."""

    figure = Cites(
        "E_O = (R_I + R_O) / R_I x Eref. Gives wide range of very stable "
        "reference voltages.",
        document="SBOA092B, Handbook of Operational Amplifier Applications",
        locator="page 52, Presettable Voltage Source",
    )

    cell = Chooses(
        "What is Eref?",
        selected="a saturated Weston cell, 1.0183 V, so R_I = 1000 x Eref = 1018.3 Ohm",
        alternatives=[
            {
                "option": "a 1.2 V bandgap",
                "reason": "the page's other circuits are standard-cell circuits",
            },
        ],
        rationale=("the figure labels the cell Eref and gives no value",),
    )

    dial = Chooses(
        "Where is the decade box set?",
        selected="8981.7 Ohm, for E_O = 10.000 V",
        alternatives=[
            {
                "option": "a round 9000 Ohm",
                "reason": "gives 10.0183 V; the dial exists to land on a round output",
            },
        ],
        rationale=(
            "the figure draws a decade box and no setting",
            "10 V is the reference such a circuit is usually asked for",
        ),
    )

    decade_box = Chooses(
        "How is a decade box drawn in parts?",
        selected="a potentiometer with its wiper tied to its far end, set to full travel",
        alternatives=[
            {
                "option": "a fixed resistor",
                "reason": "a run could not turn the dial without editing the program",
            },
        ],
        rationale=(
            "a rheostat is a potentiometer with its wiper on one end",
            "the bench can override a potentiometer's resistance for one run",
        ),
    )

    scale = Calculates(
        "i_set = Eref / R_I, with R_I = 1000 x Eref",
        inputs=("e_ref", "r_in"),
        result=(
            "1 mA whatever the cell, so E_O = Eref + R_O x 1 mA: each ohm on "
            "the decade box is a millivolt above Eref"
        ),
    )

    e_out = Parameter("V", default=10 * V, description="E_O")
    i_set = Parameter("A", default=1 * mA, description="the current through R_I and R_O")
    i_cell = Parameter("A", default=0 * A, description="what flows through the cell")

    e_ref = Cell(voltage=Decimal("1.0183") * V)
    r_in = Resistor(resistance=Decimal("1018.3") * Ohm)
    r_out = Potentiometer(resistance=Decimal("8981.7") * Ohm, setting=1 * ratio)
    amp = OpAmp(input_offset=1 * uV)
    out = Terminal()
    out_return = Terminal()
    ground = Ground()

    def architecture(self):
        # The node the cell's + terminal, R_I and R_O share.
        self.e_ref.p1 >> self.r_in.p1
        self.r_in.p1 >> self.r_out.end_a
        self.e_ref.p2 >> self.amp.inverting.signal
        # The decade box: its wiper on its far end, so all of it is in circuit.
        self.r_out.wiper >> self.r_out.end_b
        self.r_out.end_b >> self.amp.output.signal
        self.amp.output.signal >> self.out.probe
        self.r_in.p2 >> self.ground.node
        self.amp.non_inverting.signal >> self.ground.node
        self.out_return.probe >> self.ground.node

    def constraints(self):
        require(
            equals(
                self.e_out,
                product(
                    over(total(self.r_in.resistance, self.r_out.resistance), self.r_in.resistance),
                    self.e_ref.voltage,
                ),
            )
        )
        # The handbook's sizing rule, and what it buys.
        require(equals(self.r_in.resistance, product(self.e_ref.voltage, over(1000 * Ohm, 1 * V))))
        require(equals(self.i_set, over(self.e_ref.voltage, self.r_in.resistance)))
        require(equals(self.i_cell, 0 * A))


BENCH = Bench(
    page=52,
    title="Presettable Voltage Source",
    runs=[
        Run(
            "ten_volts",
            OperatingPoint(),
            measure={
                "e_out": "v({out.1})",
                "i_set": "v({r_in.1}) / 1018.3",
                "i_cell": "i(v1)",
            },
            claims=[
                Claim(
                    "e_out",
                    "e_out",
                    within=1e-4,
                    unit="V",
                    note=(
                        "Held to 100 ppm: a noise gain near 10 costs 10 ppm "
                        "of loop-gain error, and the 1 uV offset another 1 ppm."
                    ),
                ),
                Claim("i_set", "i_set", within=1e-4, unit="A"),
                Claim(
                    "i_cell",
                    "i_cell",
                    within=1e-9,
                    absolute=True,
                    unit="A",
                    note="Held to 1 nA, absolute: the model's - input draws no bias current.",
                ),
            ],
            units={"e_out": "V", "i_set": "A", "i_cell": "A"},
        ),
        Run(
            "five_volts",
            OperatingPoint(),
            settings={"r_out": {"resistance": 3981.7}},
            measure={"e_out": "v({out.1})"},
            claims=[
                Claim(
                    "e_out",
                    5.0,
                    within=1e-4,
                    unit="V",
                    note="The dial at 3981.7 Ohm: 3981.7 mV above Eref.",
                )
            ],
            units={"e_out": "V"},
        ),
        Run(
            "dial_at_zero",
            OperatingPoint(),
            settings={"r_out": {"resistance": 0}},
            measure={"e_out": "v({out.1})"},
            claims=[
                Claim(
                    "e_out",
                    1.0183,
                    within=1e-4,
                    unit="V",
                    note="The dial at zero: the lowest E_O the circuit gives is Eref.",
                )
            ],
            units={"e_out": "V"},
        ),
    ],
)

The files it writes

The parts, then the nets and the pads on them.

GND1   Ground           -
R1     1018.3 Ohm       -
RV1    Potentiometer    -
TP1    Terminal         -
TP2    Terminal         -
U1     OpAmp            -
V1     1.0183 V         -
Net-(GND1-Pad1)          GND1.1 R1.2 TP2.1 U1.IN+
Net-(R1-Pad1)            R1.1 RV1.1 V1.+
Net-(RV1-Pad2)           RV1.2 RV1.3 TP1.1 U1.OUT
Net-(U1-PadIN-)          U1.IN- V1.-

Every check that ran, and every one left undecided.

4 checks, 0 failed, 0 undecided

What the elaborated graph contains, by entity kind.

    1  block
    1  calculation
    7  component
   18  connection
    4  constraint
    3  decision
    1  evidence
    3  interface
   13  pin
   13  port
   64  total
snapshot sha256:8c24ff5f503fd7003f8f06734f6ecda5906e666a85c982f37852b57367dd2063

All of it, including the KiCad netlist, is in examples/ti_opamp_handbook/references/presettable_voltage_source/out/. Rebuild it with:

fang build examples/ti_opamp_handbook/references/presettable_voltage_source/presettable_voltage_source.py