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Sund's T100 is the cheapest differential amplifier you can find. Unfortunately, there are some snags with this little device, which is why we cannot recommend the purchase. |
Background information on differential amplifiers
What is a differential amplifier?
The name fully explains its function. A differential amplifier has two very high-impedance inputs that you connect to two points A and B that are at a voltage relative to ground. The device calculates the voltage difference A - B and puts it on its output.
Measuring between two points in a circuit
If you want to use your oscilloscope to measure the voltage difference between any two points in a circuit, you need to use the two channels of your oscilloscope. Connect both inputs A and B to those two points and set the oscilloscope to 'Difference measurement A-B'. Your oscilloscope will then calculate the difference between the magnitude of voltage A relative to ground and the magnitude of voltage B relative to ground and plot this difference voltage on the screen.
The disadvantage of this system is that you have to use both channels of your oscilloscope and there is no channel free to put a second signal on the screen. The figure below shows how, by using a differential amplifier, you only need one channel of your measuring instrument to measure the same thing.
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| Using a differential amplifier. (© 2024 Jos Verstraten) |
Suppose you need to measure the base/emitter voltage of a transistor. You connect both inputs A and B of the differential amplifier to the base and emitter and connect the output of the device to one of the inputs of your oscilloscope. As can be clearly seen, both the base and emitter are now measured via the high resistances of the inputs of the differential amplifier, and they hardly notice this extra load.
Measuring in circuits directly connected to the mains
Sometimes you cannot avoid measuring in a circuit that is galvanically connected to the mains voltage. Consider, for example, triac circuits that regulate the power supplied by the mains voltage. There is then an electrically conductive connection between each point of that circuit and the pins of the mains plug. If you connect the ground of your measuring probe to such a circuit, this point may be connected to the mains live via a very low impedance. The result is a short circuit, because the ground of the measurement probe also is connected to the metal chassis of your oscilloscope and to the earthing of the mains plug. If this does not happen, because your oscilloscope is powered via a non-grounded plug, the metal chassis of your oscilloscope is at a life-threatening voltage and touching a metal button is enough to get a severe shock.
Measuring with your oscilloscope in circuits directly connected to the mains is therefore life-threatening. If, for whatever reason, you cannot use an isolation transformer, you MUST work with a differential amplifier. How that works is suggested in the figure below.
Insert probes A and B between the measuring circuit and your oscilloscope and all problems and life-threatening situations are gone in one fell swoop. The high input resistances of the differential amplifier ensure adequate separation between the phase voltage, your measuring equipment and yourself.
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| Measuring in circuits directly connected to the mains. (© 2024 Jos Verstraten) |
Measuring currents in a circuit
A differential amplifier is indispensable for measuring currents in a circuit. Just look at the figure below. You put a small resistor in series with the device whose current you want to see. You connect the differential amplifier across this resistor. It follows from Ohm's law that the voltage across the resistor is directly proportional to the current through the resistor, so the image you see on the screen of your oscilloscope is an exact representation of the flow of current.
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| Measuring the current drawn by a circuit. (© 2024 Jos Verstraten) |
Expensive equipment
Unfortunately, most differential amplifiers are prohibitively expensive. The overview below shows the current prices of the best-known differential amplifiers:
- Hantek HT8050: € 145.00
- Micsig DP10007: € 136.00
- Micsig DP20003: € 284.00
- Pintek DP-50: € 429.00
- Testec SI-9001: € 450.00
- Testec SI-9010: € 980.00
When, while googling, we came across several deals on a differential amplifier that sells for around sixty euros, our interest was naturally triggered and an order was placed in no time.
Introduction to the T100 from Sund
Manufacturer, suppliers and prices
No manufacturer is mentioned in any advertisement, but the cardboard box reads 'Sund', brand name of the 'Shenzhen Sund Intelligent Technology Co,. Ltd.' Internet searches for that manufacturer yield the knowledge that a lot of intelligent technologies are present in Shenzhen, but Google searches for ‘Sund’ in vain. However, the device itself is very well known and is offered by almost all well-known Chinese mail-order companies and Amazon. You will pay between € 58.39 and € 114.03 for it. We bought our copy via AliExpress from the 'Daily Supplies Town Store' for € 74.20, including transport and insurance.
The scope of delivery
The little device comes neatly packaged in a sturdy cardboard box, along with a 50 cm BNC to BNC cable, a BNC short-circuit plug and a one-page manual. With that plug, you can short-circuit one of the inputs, losing the differential nature of the device and turning it into an ordinary ground-referenced amplifier.
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| The attractive packaging of Sund's T100. (© 2024 Jos Verstraten) |
The appearance of the T100
This differential probe looks very different from its much more expensive rivals. This has its reasons, which we will come back to later in this story.
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| The T100 in its small enclosure. (© Banggood) |
The T100 viewed from all sides
In the photo montage below, we have brought together all the views of this differential amplifier. What immediately stands out are the small dimensions of the aluminium case. At 85 mm by 50 mm by 20 mm, the T100 is definitely the lilliputian among differential probes and amplifiers. A second striking feature is that the two inputs are equipped with BNC connectors. Very unusual! Conveniently, the device has a built-in battery, which you can charge via a USB-C connector from a 5 V power supply. Charging is monitored by a red LED, which goes out the moment the battery is fully charged. On the back, you will find the BNC output connector, the USB-C connector and a push button switch to switch the device on. Next to the red charging LED is a green counterpart on the front panel, which lights up very modestly when you switch the device on. A slider switch allows you to set the amplifier's gain to x1, x10 and x100.
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The T100 viewed from all sides. (© 2024 Jos Verstraten) |
The technical specifications of the T100
According to the manufacturer, the T100 has the following specifications:
- Housing: aluminium alloy
- Single input voltage: -10 V to +10 V
- Differential input voltage: -10 V to +10 V
- Single input resistance: 2 MΩ
- Differential input resistance: 4 MΩ
- Output voltage: -10 V to +10 V
- Amplification A: 1x, 10x, 100x
- Bandwidth:
A = 1x: 10 MHz
A = 10x: 10 MHz
A = 100x: 2 MHz
- Common mode rejection ratio: more than 80 dB
- Noise: 5 nV / 1 kHz
- Power supply: built-in lithium polymer battery 380 mAh
- Power consumption: ≤ 0.2 W
- Charging interface: USB-C
- Battery charge time: approximately 5 hours
- Dimensions: 85 mm x 50 mm x 20 mm
- Weight: 112 g
Important note
All more expensive differential amplifiers have a differential voltage range high enough to put the mains voltage directly between the two inputs without damage. To achieve that, frequency-compensated voltage dividers are present, built from the series connection of several hefty resistors, bridged by capacitors that may or may not be adjustable. That series connection has the effect of making the PCB long and narrow. So typical differential probes all have rather long and rather narrow housings. Those devices also do not have a built-in amplifier, but a built-in attenuator. The T100, with its voltage range of only ±10 V, does not have that series connection of large resistors. Hence, it is possible to fit the electronics onto a small PCB. If you want to measure higher voltages with this device, you have to work via two 1/10 attenuator probes. These are supplied with every modern oscilloscope, so no doubt you have them in your possession.
The electronics in the T100
Liberating the PCB
The aluminium housing consists of two parts that interlock and are held by eight small screws in the front and back. After unscrewing those parts and removing the nuts on the three BNC connectors, the housing falls apart into four parts and you have the naked PCB in front of you.
What is immediately noticeable is that there is no component on the PCB to protect the device from excessive voltages on the inputs. We will come back to that. Moreover, all the resistors around the instrumentation amplifier used as a differential amplifier are so small that it is absolutely not recommended to measure at high voltage with the T100!
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| The component side of the PCB. (© 2024 Jos Verstraten) |
You distinguish six ICs on the PCB. Five of them are used to derive from the battery voltage two symmetrical voltages that feed the actual differential amplifier:
- LM337 (negative voltage stabiliser)
- MAX660 (voltage converter according to switched capacitor principle)
- LMC7660 (idem)
- SGM3209 (inverter from positive to negative)
- TP4056 (Li-ion battery charger)
The INA849
Next to the slide switch to set the gain is the only IC that has anything to do with the T100's function. That is an INA849, an amplifier from Texas Instruments. It is advertised as an 'ultra-low noise broadband instrumentation amplifier'. The figure below presents the internal block diagram of this circuit.
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| Block diagram of the INA849. (© Texas Instruments) |
This amplifier has superior specifications:
- Noise: 1 nV/√Hz
- Bandwidth: 28 MHz (A = 1), 8 MHz (A = 100)
- Slew rate: 35 V/µs
- Common-mode rejection ratio: 120 dB
- Very high input impedance: >100 GΩ
- Gain adjustable with one external resistor
The circuit has a current-feedback topology responsible for its wide bandwidth. Using one external resistor, you can set the voltage gain between 1 and 10,000.
Another reason not to play with high voltages!
The PCB slides into two slots provided in the bottom of the housing. Between this underside and the furthest protruding solder blobs on the PCB is a distance of less than one miliimeter! For such a device, this is an inexcusable design flaw. So we warn again: do not use the T100 to go measuring in circuits where high voltages are present.
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A rather tight distance between the PCB and the enclosure. (© 2024 Jos Verstraten) |
Testing the T100 from Sund
Measuring the differential input resistance
According to the specifications, it should be 4 MΩ. However, we measure much lower values, when switched off, which depend on the measuring device we are measuring with. This intrigues us and hence we dive a little deeper into the PCB. Then we make a shocking discovery: the two inputs of the INA849 go directly to the two BNC connectors, without any measure against excessive voltages on the inputs. True, the INA849 does contain internal protection diodes to both supply voltages, but this design philosophy, in our humble opinion, deserves absolutely no beauty prize.
Because the INA849 itself has a very high input resistance and the two inputs must have a path to ground, two external SMD resistors coded '30E' have been fitted. That is the EIA-96 coding for '200' with multiplier 'x 10 kΩ'. The resistance value is 2,000 kΩ and so the T100 indeed has a single input resistance of 2 MΩ and a differential input resistance of 4 MΩ. However, this cannot be measured when switched off with a multimeter, but is only present when the INA849 is active.
A second shocking discovery is that the BNC connector marked 'IN+' goes to the negative input of the INA849 and the one marked 'IN-' to the positive input. So the texts on the front panel are wrong!
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| Schematic of the input circuit of the T100. (© 2024 Jos Verstraten) |
Measuring the noise of the T100
We short-circuit the two inputs and connect our mV meter PM2454B to the output. For the three amplifier settings, we measure the average noise voltage shown below:
- position '1x': 0.89 mV
- setting '10x': 0.95 mV
- setting '100x': 2.05 mV
In itself, these are low values, but they are much higher than what the specifications promise. The oscillogram below shows the shape of the noise voltage in mode '1x'.
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| The T100's noise. (© 2024 Jos Verstraten) |
Phase shifted!
The wrong indication on the front panel results in a phase rotation of 180° between the signal on the 'IN+' and the signal on the output.
Note
In all the following oscillograms, the blue traces represent the input signal and the yellow traces represent the output signal.
Testing the frequency range at 5 V in the '1x' mode
We test this by applying a sinusoidal signal of 5 Vrms between the two inputs and putting the input and output voltages of the T100 on the screen of our oscilloscope. We start at 1 kHz (upper left) and go up to 2 MHz (bottom right). At 1 kHz and 500 kHz, the input and output signals are nicely the same size and in phase. Clearly, at 1 MHz, the output already has trouble following the input signal. At 2 MHz, nothing remains of the input signal. So the promised bandwidth of 10 MHz is definitely not achieved.
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| Frequency range in '1x' mode and at 5 V input signal. (© 2024 Jos Verstraten) |
Testing the frequency range at 1 V in '1x' mode
A peak-to-peak voltage of 2.82 ● 5.0 V = 14.1 V might be a bit too large to judge the bandwidth. Hence, we repeat the measurements with a voltage of 1 Vrms, with frequencies of 1 MHz, 5 MHz and 10 MHz. The results are summarised in the figure below and are rather disappointing. At 1 MHz, the output faithfully follows the input, there is only a slight phase shift. At 5 MHz and 10 MHz, the output signal is heavily distorted and there is substantial phase shift.
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| Frequency range in '1x' mode and at 1 V input signal. (© 2024 Jos Verstraten) |
Testing a square wave signal of 500 mV in '1x' mode
We put square wave signals of 500 mV and with frequencies of 1 kHz, 100 kHz and 1 MHz at the input and again observe the output voltage of the T100. The results are summarised in the figure below. To avoid cable reflections, both inputs of the oscilloscope are terminated with 50 Ω terminators. As a result, the output signals from both our function generator and the T100 are attenuated, which is why the sensitivities of the two channels are not at the same setting.
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| Display of square voltages in the '1x' position. (© 2024 Jos Verstraten) |
Testing square wave signals of 50 mV in '100x' mode
Sine wave amplification at 1 kHz and at '10x' and '100x'
In the '100x' mode, the T100 should amplify the input signal by a factor of one hundred. So a square voltage of 50 mV should appear on the output as an identical signal with an amplitude of 5.0 V. This turns out to be quite correct, but to avoid cable reflections, we again terminate both inputs of the oscilloscope with 50 Ω terminators. This makes it seem as if the T100 does not amplify the signal a hundred times, but it does. The measurement results are shown in the figure below. As might be expected (specified bandwidth 2 MHz), the pulse response at 1 MHz appears even worse than in the '1x' position.
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| Display of square wave signals in the '100x' mode. (© 2024 Jos Verstraten) |
Sine wave amplification at 1 kHz and at '10x' and '100x'
In the following measurement, we investigate how well (or badly) the T100 amplifies a very small sinusoidal signal with a frequency of 1 kHz in the '10x' and '100x' modes. As a source, we use a sine wave signal of 3 mVrms. That voltage must therefore be amplified to 30 mV and 300 mV, respectively. You can see the results in the following photomontage. At the '10x' setting, things go well, except that a small phase shift can be seen. At setting '100x', the output voltage shows severe distortion. In these measurements, we accidentally swapped the two inputs, hence the 180° phase shift between the input and output.
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| Display of 1 kHz sine wave signal in '10x' and '100x' modes. (© 2024 Jos Verstraten) |
Our opinion of Sund's T100 differential amplifier
The device looks nice, the price is good and the all-metal casing makes a professional impression, but that is the only positive thing that can be said about it.
The design contains serious flaws. That the texts on the front panel are wrong is incomprehensible. Equally hard to understand is the unprotected circuit around the inputs of the INA849 and the fact that the solder on the PCB almost touches the housing.
It follows from the measurement results that the T100 does not meet any of the manufacturer's stated specifications. Usually, we sell a device we have tested back to a hobbyist at a soft price. With this little device, we are not going to do that, because we would feel like a crook. So those in need of a differential probe will still have to dig deeper into their pockets and buy one of the devices from Hantek, Micsig or Testec.
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