Can ULN2003 be a poor-man’s matched diode array?
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I have a log/anti log application that requires “slightly” matched diodes – something better than you get just by grabbing a bunch of 1N4148s from a bin. I need the matching to be across each group of 3 diodes. The goal is to be cheap, the relative accuracy of the circuit is somewhat loose – +/-20% Vf-vs-I match is plenty good.
All the diodes are continuously forward biased and operate over a 2 decade current range. The bandwidth is <1kHz.
I was wondering whether the flyback diodes inside of an ULN200x family of chips could be used for this purpose, assuming that otherwise the chip’s absolute maximum ratings won’t be exceeded (power dissipation, diode current, etc.).
My presumption is that they’d be better matched initially, and better thermally tracking, than a bunch of discrete diodes on a common heat spreader, say several leaded SMD diodes – not selected/matched – mounted close together on an aluminum substrate board.
I will likely post my own measurements, when I get a chance, as an answer – but I was looking for any suggestions/input anyone might have – as well as any alternative chips that could be conscripted for this purpose.
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asked 20 hours ago
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- 2I have seen this in a professional testing equipment, and was surprised about the idea, but I don’t know the engineering background of this decision. I only remember a low leakage current. – Jens 20 hours ago
- I used to buy didoes in lots of 10 to 100, and they usually came in “sticky paper stripes”. I guess it is very likely that the didoes stuck on each same stripe are from the the same chip/wafer, so should be very electrically matched. Reference: AliExpress 14 Values Diode Kit, 350 Pieces, 1N4001, 1N4004, 1N4007, 1N5404, 1N5406, 1N5408, RL207, 1N5817, 1N5819, 1N5822, UF4007, FR107, 1N4148 HKD96.63/lot(350 pieces ) / to continue, … – tlfong01 19 hours ago
4 Answers
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I’ve measured thermal tracking on SOT23-6 dual matched transistors like DMMT3904, by sending a current pulse in one transistor and measuring the Vbe of both. Thermal coupling is very poor. They’re two independent transistor dies, not monolithic, and the only thermal link between them is the plastic package. So they’ll be at the same temperature only if ambient air heats the package, but not if they’re self-heating or heated differently through the pins.
So you’d need your diodes to be on the same chip, a monolithic array, for matching and thermal tracking. Using ULN2003 is a nice idea.
There are also Transil diode arrays that might fit the purpose. Series resistance is very low due to the huge max current, but other diode characteristics like leakage and capacitance may not be adequate. This one seems to be monolithic (the datasheet mentions it). There are other tiny flip-chip multichannel ESD protection diodes where you can be sure it’s monolithic because you can actually see the chip.
There are simple diode arrays but it is difficult to know if they’re monolithic or just some diode chips on a common leadframe. That would still give good thermal tracking, but not matching. For example this one‘s package hints that all the diodes are on the center thermal pad, but it doesn’t say whether they are independent diode dies or monolithic.
I’m interested in what you find…
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answered 17 hours ago
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It should work. You might have to connect the GND pin to a negative bias point to keep sneak paths from conducting.
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answered 19 hours ago
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While the datasheets don’t specify any of these diode characteristics, at low currents (say < 10 mA), where the resistance of the metal interconnect won’t come into play, they will likely match extremely well.
You should characterize some to see over what current range they will follow the exponential law — at low currents, they may are leaky; at higher currents, some parasitic series resistance may have an effect. In addition, you might find that the VF of the diodes does depend on the VCOM voltage w.r.t. GND.
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answered 20 hours ago
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- Excellent suggestions – thank you! – Kuba hasn’t forgotten Monica 11 hours ago
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The CA3046 transistor array can be used to provide a minimum of 4 B-E diodes. This is a monolithic die which guarantees the diode matching to 5mV, and the thermal matching should be excellent.
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answered 9 hours ago
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- My concern is that while that chip is available second-hand and is made again by a factory in Ukraine IIRC, it’s not a part you can just buy from DigiKey. Good for retro stuff in small quantities, though. I was trying for a solution that’s easier to buy legit without worrying about counterfeits or large minimum quantities from Rochester. There is HFA3046… from Renesas, for $12 apiece. Nuts. – Kuba hasn’t forgotten Monica 3 hours ago
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